The question is not how many graves there would be, but rather how many graves are currently known to the inhabitants. Often graves of non-nobles are marked with wood posts / crosses or nothing more than a mound - perhaps with a tree or bush next to or on it as a marker. After three-four generations how likely is it that the post is still standing, the mound not leveled from the many passages of feet and the rain, the significance of the bush marker remembered - all by the great great grandchildren of those forgotten graves?
Add on another generation or two, and unless a stone marker was used (and sometimes even then if it was not cared for, as it could have fallen down and been buried in earth and vegetation - perhaps even shattered by roots of a nearby tree) it is likely that the grave no longer has any marker to show it exists or even ever existed. To the current generation, this region of the graveyard is unused land, obviously set aside in pre-memory times by ancestors who knew the graveyard would eventually need room to expand. As the placements are often barely organized, it is hit and miss as to whether the digging of a grave will unearth a skeleton from a couple hundred years ago - and if it does what will the grave digger even think of it? Obviously a grave was once there, but the grave was so old that no marker existed, and none know when it was dug, who might have been in it, etc. It will be returned to the consecrated earth - ie: tossed back into the grave with the grave dirt when the coffin is buried. And note that the bodies were often buried in clothing, a shroud (cloth to cover them), and a wooden box just large enough to fit the corpse. All of this is highly biodegradeable. By the time a few bones are found while digging a grave likely only a few chips of wood are left of any prior coffin - if that.
So, I imagine that only the last 3-4 generations are ever represented in most common graveyards, with noble graves perhaps stretching back twice that far - with a few exceptions going back even farther, but such would be found in special crypts, on monestary or cathedral grounds, etc. Thus a graveyard might start to the left of the church, and a few generations later (due to overflow) move to the right of the church. A few generations later the left is 'open' again and the right is starting to need overflow towards the left. Something like this:
Code:
Start (O: empty, 1,2,etc: filled)
O O O O O O O O
O O O O O O O O
O O O ### O O O
O O O ### O O O
O O O #+# O O O
Time Passes (gen 1 dies)
O O O O O O O O
O O O O O O O O
1 1 O ### O O O
1 1 1 ### O O O
1 1 1 #+# O O O
Time Passes (gen 2 dies)
2 2 2 O O O O O
2 2 2 2 O O O O
1 1 2 ### O O O
1 1 1 ### O O O
1 1 1 #+# O O O
Time Passes (gen 3 dies)
2 2 2 3 3 3 3 3
2 2 2 2 3 3 3 O
1 1 2 ### O O O
1 1 1 ### O O O
1 1 1 #+# O O O
Time Passes (gen 4 dies, gen 1 forgotten)
2 2 2 3 3 3 3 3
2 2 2 2 3 3 3 4
O O 2 ### 4 4 4
O O O ### 4 4 4
O O O #+# 4 O O
Time Passes (gen 5 dies, gen 2 forgotten)
O O O 3 3 3 3 3
O O O O 3 3 3 4
O O O ### 4 4 4
5 5 5 ### 4 4 4
5 5 5 #+# 4 5 5
So, as you can see, it is something like unintentional crop rotation. For a zero-growth society there is always space available, for a negative growth one this is even more true. Positive growth was something of a rarety through most of the Middle Ages. Rarely did any location not experience a notable negative growth period in any given century - enough to significantly slow if not negate any recent "growth." The Middle Ages had so many famines, plagues, etc - especially minor local ones - that it was all but a certainty that eventually (out of no more than four-five generations) a manor or village would suffer one. The number of graves - especially poorly marked ones - would soar, and entire families might be wiped out. A couple generations later the graves of prior members of wiped out families will have been lost / forgotten, and the stigma of not using lands due to mass burial of famine victims might even be waning towards forgetfulness (mass burials of disease victims were typically remembered a few generations long - usually; the black death returned at least twice in minor epidemics due to the forgetting of where a mass grave of a mere one or two hundred years ago had been placed while expanding the building of a town).
Now, the situation is a little more complex than what I showed above, I readily admit. Some older graves might be recalled for 6-10 generations, while some minor ones might be forgotten / lost after as little as 2-3 generations. So the 'openning' of land for new graves was spotty rather than vast - more akin to the below:
Code:
O O O 3 3 4 3 O
2 2 5 O 3 O 3 4
O 5 2 ### 3 4 4
O 1 5 ### 4 4 O
5 5 5 #+# 4 5 5
Also, some would want to be buried in specific locations - perhaps near the church, perhaps next to a tree (that might have been a sapling for a forgotten relative a couple hundred years back, all unknown to the recently deceased), etc. Also, it would not be in neat rows that the text suggests. What was a path between graves 150 years ago might have become a new grave, the graves between unknown yet undisturbed. After 500 years or so the ground is a mix of of hundreds of skeletons - all at about the same depth (give or take a couple feet), most in fragmentary or mixed condition due to spades breaking old bones and the return of a bone to the earth with grave dirt often missing (or not even looking for) the rest of the skeleton.
This does not even take into account that grave robbers were - in some locatioins - a real concern. They might come a few days after a wealthy merchant or noble is buried in a poorly guarded cemetary (and except in towns that had necropolises or crypts or otherwise specially designated grave areas cemetaries were almost always poorly guarded) to dig up the body, removing any rings, bracelets, necklaces, etc - even the clothing if it was made of valuable material readily washed to remove signs of its most recent use. Then the rest is reburied - well, tossed in a more or less reburied. If the grave is fresh enough the digging might even not go noticed. Animals, also, might occationally dig up bones and bodies. It is thought that most legends of ghuls started this way.
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
All in all, the cemetary will not likely hold all that many actual
known graves. Perhaps a hundred or - at most twice that number. And likely as many as a quarter will be overgrown or otherwise barely visible or known, with a single area perhaps well kept and holding the most recent graves - the most recent couple generations.
There may indeed be 500 bodies in that cemetary, but the village will only know of a quarter to a third of that number.
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Taking into account some other concerns:
True, the population was certainly not 150 continuously, but an 8% growth rate is unlikely in the middle ages - even the late middle ages. It will be better than that of nomadic or semi-nomadic tribesmen (which, as I recall, is around 1-2% growth rate), but there will also be times of negative growth. As I stated above, rarely did a century go by without a famine or epidemic striking a locality - and I mean
only a locality.
I recall reading that while a noble might have a reasonable expectation of how much wealth will come from a given set of lands, it was often understood that while these two manors over here are having notable time of plenty, these other two over here are likely experiencing a couple years of famine. Due to the difficulty of moving large amounts of food over any real distance, it was not expected that much could be done for them. Some would die of starvation, even as others a mere hundred miles away were eatting more than their typical fill every day. All in all it (usually) balanced out for the noble, as a few years later the situation for the two example regions might have reversed - or perhaps those two stabilized but two other areas were experiencing famine / plenty.
So, overall, it is possible that the village has had - more or less - around 150 population the entire period. It may, over the last 200-300 years, have fallen as far as 120 or risen as high as 180, but over all its population has been around 150. Perhaps it started around 120 and is now - 250 years later - 150. During the intermission it could have risen to 200 or fallen to 50. A famine can easily slay half the population, and a few years of plenty can easily restore that number. So long as the famines are uncommon the population can bounce back and even prosper, being affected by the famine more like a plant being pruned than a plant starved of water or light.
As I already stated, however, this is mostly immaterial. For a typical and common middle ages village it is unlikely that the graves will count more than the prior 2-3 generations, with a few notable 4-5 generation plots. Maybe one area has a single stone and is said to signify the grave of the town founder and his family. They are not quite certain
where the grave it, but they know it is in that general location, and no new plots can be dug there, despite no grave markers other than the single rock - which may have been placed there based on rumor or a chance dig up of a grave a couple generations back. So, I would guess that for a settlement of 150, no more than 150-200 graves exist in their cemetary, and likely fewer than that. But certainly more than 50, and likely more than 100.
