People will tell you different things, but if you want to get competitive then having one main will definitely make the learning process easier. There are 26 characters, so learning 25 match ups for a single character is daunting enough.
Though I'm not one to talk, I use about 5 characters because I'm too indecisive to pick a main and there are still match ups that seem a bit foreign to me, so don't follow everything I'm saying like I'm a pro or anything! :P
But yeah, most people will tell you that dedicating your time to one character will allow your competitive game to grow more quickly, while there's no harm in having a secondary or two for a bit of variety and to excel in the match ups that your main struggles with. It's totally up to you though!
Oh and I know all this stuff seems a bit overwhelming at first, but bear in mind that there's only so much you'll instantly learn from the theory and reading up on frame data. You'll build your skills with experience and by gradually transferring the stuff you learn here into your game. Fighting people better than you always helps too as long as you aim to learn something from the losses :)
