STATISTICS OF THE POPULATION
Let us analyze general data though they differ.
In 1856, the population of Russia was 74.556 million people (Wikipedia. Census revisions)
In 1859 the population of Russia was 67.081 million people (A.G.Trojnitskiy «the Serf population of Russia according to the 10th national census». А.Г. Тройницкий «Крепостное население России по 10-й народной переписи», 1861 г.)
The difference is 7 million, that is about 10 %.

We already saw that antique slaves, strictly speaking, were not slaves as they did not lose legal capacity and lived according to the same legal norms, as free people. Now we will consider serfs.1762, 1794 SERFS WERE NOT SOLD
Friend alexandr_tx3 proposed a hypothesis that sale of peasants could be sale of labour duty, that is, work sale, not people sale.Only one scan of bill of serfs’ sale was found for the period of 1832-1861. It was for 1844, moreover, impossible to read and without decoding. However, there were numerous inquiries about collecting of arrears from serfs for 1855-1858. The problem is, those arrears mean that the serf can be in debt, and the debt means legal personality: a thing can not be in debt.