Families - average count two adults and 2.5 kids, the problem with this is that poor have more kids and other family members living with them, so you will have high density of people in some areas.
Prior to the modern era, this is what census takers actually tried to count. But you have tons of anachronisms in that.
First, families and taxpayers were basically the same thing. You counted families to count taxpayers. The head of family may have owed the tax, but the whole family was effectively liable for it. There was no such thing as a youth or poverty exemption. If you weren't a family, and weren't a taxpayer, you were a legal criminal - a bandit, beggar, vagrant or the like to be put lashed, pilloried, imprisoned and put in the work houses. If you were lucky, a monastery might take you in and put you to work, but that was about the whole of the social safety net that existed. As a largely subsistence economy, that's all they could afford. Attempting to feed you if you couldn't be made productive would have meant someone else was starving. There was barely enough left over food in the good years to feed your widowed grandmother or aunt, or your orphaned nephew.
On average, women would give 7-8 live births. Eventually, this would kill most of them. But the life expectancy on men or children was hardly better. Of the 7-8 live births, maybe 3 or 4 would survive to adulthood. And even so, a third of children would see one or both parents die before they reached it. Those Grimm fairy tales filled with ache and horror and broken homes and dreams of some sort of rescue - that was the reality of daily life. You'd lose your parents, and you'd quite probably enter a world of complete social and economic insecurity, abuse and exploitation. Childhood as you understand the concept was invented by the Victorians as enough wealth arrived to support it. Prior to that, by age 10 you were working in some capacity - field hand, maid, apprentice, etc. - not just because your family needed the income, but because you needed to get independent and get some economic value attached to you as soon as possible in case you lost your family. Poverty was indigenous to subsistence pre-industrial agrarian economies in a way you can't imagine.
All this meant that a household looks nothing like a modern American urban family. A typical household might be a grandfather or grandmother, 2 adult children and their spouses (which might be a second spouse, the first having died in childbirth), 1 unmarried adult child, plus 3-8 living grandchildren the oldest of which might be approaching marriageable age. Taxes were owed on account of the family land, whether freeholders, cottagers or serfs. The main differences were your legal rights, because all people weren't born equal in the eyes of the law.
This would be even more true of the wealthier households who'd have more children and more family members living them, because children were a sign of wealth and not poverty (and likewise an indication of the families future economic success). It was poor families who had fewer children, fewer resources to support them, greater losses due to malnutrition and starvation and disease and ill-health resulting from short food supply. It was poor families that couldn't afford to stay together as a family unit and who more often lacked surviving family members to depend upon for protection. A farm with lots of children on it was one that was wealthy, not just because those children were farm hands or hand secondary incomes as maids or apprentices, but because it meant the parents were healthy (and often the children of healthy parents). Families would take great care who to marry, because marriages were primarily economic contracts between families. Join yourself to an economically poor partner might mean your children or yourself would have no support in the event of one or both partners death.
Equality - are all counted the same or does it take four gnomes & Halfling to equal one human?
Well, you don't have voters. You are taking a census to know how much tax each lord owes his lord. You aren't really counting persons, you are counting tax. If Halflings and gnomes are as economically productive as humans, then they pay tax accordingly. Otherwise, they still get counted, they just pay less or a least a different tax.
Then monsters, are they even counted?
Do they pay tax to the lord of the land? If so, yes. If they don't, then they get counted as the reason that the lord doesn't owe tax on a certain piece of property. For example, the census might record:
"Barony of Tibbleton...
And there is in the district of Beeby also a fine wood, amounting to 7000 acres, stretching from Beeby to Wallsburch. But from this wood there is no good forage for pigs and no wood can be cut, owing to the presence of a fearsome wyrm of great size and evil disposition, whose depredations are a scourge upon the wood and on the surrounding countryside."
Translation, the Baron owes his lord no taxes on account of the wood - which might otherwise be very economically profitable - because there is a dragon that renders the property values lower than an accounting of the extent of the forested area would indicate.
On the other hand, in a fantasy census we might have something like,
"And in the district of Vengard, there is a fairy wood, amounting to six hundred and three score acres. And in this wood there is a Sidhe Court, which by ancient tradition has all rights pertaining to the wood and holds it in fee complete to the King, and may not be disturbed on penalty of the King's Justice. But, in accordance to this tradition, the fief-holder of Barkindale renders tribute to the Sidhe Court twenty tuns of good wine annually, for which he is rendered a payment of 30 pound of good fine gold."
Translation, heck yeah, the Baron of Barkindale owes taxes on that wood! It's a freaking gold mine! The King better be getting part of the profits. Who cares how many fairies live in the wood, point is, they've got gold and share it!
