depends a lot on whether the town is growing in size, has suffered major catastrophies etc.
Assuming the 150 is equally spread between 3 generations and generations are approximately 30 years apart, population has been zero growth and the graveyard was started around about the time that the first generation of people who were raising families started to die then you would had between 7-8 generations die (assuming death around 50-60 year mark on average) which gives about 350-400 graves.
If it is a frontier town where people may have died and the bodies never been recovered then adjust the number down.
If the town has grown from being founded, then adjust the figure down.
If it is near a larger city/town then adjust the number down as people may have been buried back in the larger town/city.
Increase the number of graves for babies/children, and mothers of young children who might have died in childbirth - especially older mothers, men killed in war/accident (15-25) and adjust for any perculiarities.
Roughly based on the above - 250 graves
- 30 babies and children under 2 years old
- 10 children between 2 and 15
- 10 females between 15 and 25
- 40 males between 15 and 25
- 50 females between 25 and 50
- 50 males between 25 and 50
- 45 females over 50
- 15 males over 50
These will be fairly evenly spread with more baby and male (15-25) deaths occurring earlier in the period - plus baby deaths often tend to be grouped (cold winters, diseases, etc).
All of the above is total conjecture on my part, but may be of some use to you. If not then just ignore it.