This question is dangerous territory.
You can know thousands of French words. But words, by themselves, are almost useless. You need to combine these words in worthwhile patterns to create comprehensible speech. Look at this sentence in English:
*Inadvertently, wallet an superior windowing pew dynamite wrote the swam.
Would you call this person fluent? No? But, look at all those juicy words!
Spend hours a day drilling vocabulary and you can acquire 5,000 French words in a month. But if you can’t follow a native speaker, conjugate verbs, or pronounce words but then you’re just fooling yourself.
Fluency is the ability to speak and understand a language, not necessarily memorizing vocabulary. I cover how long it takes to attain French fluency. Some people focus on boosting their vocabulary but there are other important things to master first (or at the same time). Would you want to form strong sentences or puff up your speech with words French speakers don’t even use on a daily basis?
Besides, asking someone to explain a word or looking it up is easier than figuring out a foreign sentence based solely on the few words you understand.
Those who endorse a vocabulary-heavy language learning approach oftentimes attempt to sell you products that use said methodology.
While there are words that are more common in the French language, or there may exist cognates to make your life easier, do not fall into the trap of attaining a mystical number of vocabulary words or buying products that claim that [X] number of words is fluent. Vocabulary makes your life a lot easier, but I believe that once you have the fundamentals down, then, you can cram vocabulary. Know how to use a language first, then acquire the puzzle pieces to be more descriptive.
When learning French, keep functionality before fluff, and the framework before the extraneous content. – John Elkhoury