From: http://www.insidescience.org/content/massive-study-shows-how-languages-change/2096 Ever wonder why some languages are harder for adults to learn than others? The researchers point out that languages contain what linguists call a “kernel lexicon,” meaning a list of words that constitute 75 percent of the written language. If you know those words, you can make out much of the literature. These also are the words least likely to change even as the language morphs. The kernel lexicon for English is less than 2,400 words. If you know them you can read 75 percent of the text. The kernel lexicon for Russian is about 24,000 words. So, even though the whole of the English language has about 600,000 words and Russian has considerably less than half of that, without the crucial 21,000 kernel words, most Russian writing would be largely incomprehensible.

Learning a language is much more complex than just memorizing words. I don't think that it's useful and productive to learn just separate words. What is more important is trying to learn phrases and words combinations and using them in your conversation. You can memorize 10 000 words but you are not able to build sentences and speak. But if you really want to learn just words here you can see top-100 and top-2000 most commonly used russian words: top-100 http://blogs.transparent.com/russian/100-must-know-russian-words-and-how-to-learn-them/ top-2000 http://www.learnrussianfree.com/vocabulary

It isn't necessary to have a certain number of words, as more important is the ability to get your meaning across in conversations you normally partake in, on a daily basis. 5000, 7000? Think of the type of conversations and topics you have now. Friends, co-workers, school mates, store merchants, school studies, hobbies, books read, movies read. Throw-away conversations. Directions, sports teams, weather. Stuff people use to start a conversation. It all compiles into your fluency.

about 10 thousand

Russian is more difficult than I thought.