Some linguists (see this BBC article) have estimated that the average native speaker has approximately 20,000 words in their vocabulary (obviously not counting all inflections and declension for cases). Likewise the Russian ministry of education defines C2-level (native-proficiency) as having a vocabulary of about 20,000 words.
My question is, how are verbs counted? Is
делать-сделать counted as two words, or one word? This is a question about the definition of "lemma" in Russian linguistics - do Russian linguists consider two aspects of the same verb to be from the same lemma, the same word?
I would appreciate any references to academic studies in Russian on the subject of counting the number of words in the language.