Part of the reason why there is no solid estimate is that the tsarist state did not exactly keep the best records and there was not a consistency to its various censuses. For example, various poll-taxes and other methods of counting only counted male serfs and peasants; younger children and women were usually excluded from these counts. The distinction between state peasants and private serfs is also a tricky one.

The wiki page Serfdom in Russia states:

By the mid-19th century, the peasants composed a majority of the population, and according to the census of 1857 the number of private serfs was 23.1 million out of 62.5 million Russians, 37.7% of the population.

The exact numbers, according to official data, were: entire population 60,909,309; peasantry of all classes 49,486,665; state peasants 23,138,191; peasants on the lands of proprietors 23,022,390; peasants of the appanages and other departments 3,326,084.{33- dead and unknown link} State peasants were considered personally free, but their freedom of movement was restricted. { 34-goes to an odd link }

These numbers do square up with the general estimates cited by the likes of David Moon and Peter Kolchin. The latter historian's Unfree Labor cites V. M. Kabuzan for a figure of 24 million male serf and state peasants in 1858. The figure for private serfs is 11.3 million male serfs. Given the uncounted children and women, Wiki's citing of 23 million private serfs does appear to be valid.

Part of the problem here is that serfdom within Russia was a multifaceted system. The institution varied highly over the vast Russian empire. Many of the censuses focused on European Russia and ignored the Don and Black Sea Cossack areas. Historians often have to estimate the numbers for these areas. Defining who a "serf" was is also difficult in 2017 because the Russian state had different definitions of serfdom. State peasants were not considered proper serfs in the eyes of their contemporaries, while many of the conditions of the state peasants would not be all that different from a serf from a modern standpoint. State peasants still owed obligations to their social superiors and suffered under the same number of restrictions on how they would live their lives. They may have been legally free by the terms of the Russian state, but such freedom did not really amount to that much for the state peasantry.