A lot! But even with a vast vocabulary, you will need to speak, read, write and listen with skill.

There is no definition what 'fluent' means. To get by, find your way, speak about every day events, and understand basic general conversation you normally need about 3000 words. To be at a level of highschool student you probably need 6-10k. Then it depends on your speciality.

To be fluent in English, there is no specific number of words that you have to learn. You should be clear with all the grammar, vocabulary, reading and writing skills, etc.