It’s difficult to say exactly how many words there are in the Russian language, as Russian is constantly evolving. Also, it depends if you count each conjugation or case ending as a different word.
The Great Academic Dictionary of the Russian Language says that Russian has around 150.000 words. However, if you add all the dialect and slang words, we might reach around 400.000 words.
Now, that’s a lot of words if you want to learn Russian.
Luckily, you do not need to know all of them.
Even native Russian speakers will only use on a daily bases several thousand words. So you as a foreigner can get by with knowing a lot less words.
It’s been estimated that if you know the most common 2000 words, you can have most conversations to live comfortably in Russia. And if you were to learn 5 words per day, that’s about a year worth of effort.
And if you were to learn 10 new words per day, it would be half a year. So in reality, it’s not that difficult to learn Russian.