What is the enough vocabulary sufficient to speak Spanish fluently
Cherry's right. Also, you need to ask yourself how do you define fluency, because I've found that different people have different standards about what exactly it is to be fluent in Spanish. Do you want to know enough Spanish to maintain a general conversation? Or do you want to study, work or live in a Spanish-speaking environment? Those are two different things but there are many people who would be satisfied with just being able to carry on a general, basic conversation in Spanish.
If you want to give yourself a number to shoot for, try to build up a vocabulary of 1,500 to 2,000 words. I read somewhere that a vocabulary of 1,500 words is the target to shoot for when building proficiency in a language. Set a goal that you can live with...maybe learning one new word a day or five new words a week.
First build your vocabulary around everyday things that surround you or affect your daily life, like family, friends, food, work (or school), pets, the weather, time/date/seasons/holidays. Then build up your vocabulary related to expressing opinions, your likes and dislikes. Learn all the vocabulary that goes with whatever sports or hobbies you may have. Make sure you keep your vocabulary balanced by learning nouns, verbs, adverbs and adjectives. Too many people focus on learning nouns and then later on realize they don't know that many verbs or adjectives to describe those nouns.
Welcome to the forum, Miguelito. The truth is that there isn't an exact number of words that you have to learn to be fluent. Language learning is so much more than just learning vocabulary. The structural aspect of it is very involved. I would suggest that after you feel comfortable with it, you should put yourself into some type of immersion program. The language comes more quickly when you are surrounded by it day and night and depend on it to survive in everyday life. You would be in fantastic shape after spending 6 months to a year living with a Spanish speaking family that doesn't know English. You can study all of your life and might never learn every word that exists in Spanish. The important thing is to keep learning new things and practicing what you learned everyday. A good way to do this is to not only study what is taught in your class, but to think about things that are important to you. Develope a personal vocabulary list. It will be more practical and meaningful to you.