Rulers would have great interest in knowing how many subjects they ruled for the purpose of tax collection and raising armies.

It is not like one guy has to travel all around counting. I would assume that each of the great estates would keep track of how many peasants they had on their lands and report that.

Keep in mind that Russia had serfdom. The Russian aristocracy basically owned peasants.

Estimating populations when you don't have centralized states is much harder. E.g. the population of Ancient Greece is much more likely to be wrong.

In the 1700s one could expect most European states would keep quite extensive records which historians could use. If not the state aparatus one could use church books.