Did the rulers of the Piast dynasty counted inhabitants on their territory? Or were there at least smaller censes of population in towns or villages? If so, do we have acces to some of these documents?
I have found only a rough estimates of population size in this region, that seem to be based on acheological evidence, but I was not able to find a mention about any population census.
In a word, censuses really only start in the High Middle Ages, when it became necessary or desirable to have an idea of how many people lived in a city or country: one might cite the Domesday Book as a prime example. Poland was probably a late-comer to such practices if anything, as the English and French censuses were bound to specific sociopolitical contexts (determining a rough headcount of the population after 1066, reassessing taxes, etc...).
Trying to find good estimates is going to be impossible for anything pre-modern. The best you're going to get is that the estimate for 1000 AD rarely strays from 'about a million inhabitants'.
Sources:
Biller, Peter, The Measure of Multitude: Population in Medieval Thought (New York, NY: OUP, 2001).
Cesaretti et al., 'Population-Area Relationship for Medieval European Cities', PLOS One (2016).
Lukowski, J, and Hubert Zawadzki, A Concise History of Poland (Cambridge: CUP, 2001).
Russel, J. C., 'Recent Advances in Mediaeval Demography', Speculum 40 (1965), 84-101. [For older historiography]