How many words do I need to know to become fluent in Spanish?
The answer is;- the same number of words as you need to be fluent in your native language. How many words in that do you know right now? If you want to learn a language, the number of words is not important. With any common language, you will need to learn thousands, over time. It is how you use them and perceive them that determines how fluent you might be, not an abstract sum total.
It requires lots of regular practice and effort on the part of the learner, like anything else worthwhile. All you need to do is get started and keep going.
To become fluent, you need to have somewhat rich vocabulary and, most of all, you need to speak in Spanish without any problem, that's called fluency.