As with so many aspects of painting, there is no 'right' or 'wrong' answer to this question. How many brushes you use when painting, and what shape and size they are, is a matter of personal preference. You need to try painting with different various shapes and sizes, try working with several and with just one, and see what works best for you.
Personally, I like working with just one brush, specifically a No.10 size Filbert with hog or bristle hairs. By working with just one brush size and shape I really get to know what I can do with it. I don't have to think about how I get a particular mark with the brush, it's become instinctive. When the brush hairs wear down, I buy a new one, same size, same shape (though not always the same manufacturer; it depends on what I can find and the price).
Working with just one brush, which I rinse clean regularly in a jar of clean water while I'm painting, I also don't have to worry about the acrylic paint drying in it and ruining it. (Of course, if you use oil paints, which dry much slower, you can leave paint in a brush for much longer without worrying.)