I've been playing this game for years and have thousands of games played.
I prefer not having a main. Being able to play any champ at any half level is more fun and keeps the game fresh.
However, when I play ranked, I keep my champion pool very limited. There is no denying that you will perform better on champions you play frequently.
The best way to play, I find, is to create a physical list (seriously, write it down and tape it to your desk) of champions that you will play. On a base level, I recommend three champions per role. These are three champions you have a decent number of games on, have a decent power level at the moment (power picks are important). and knowing match-ups.
So like, my top lane pool is:
-Irelia
-Sion
-Cho'Gath
Irelia is a power pick, no doubt about it. She's just strong at the moment, and can swing a game.
Sion is in a good spot, and provides a lot of CC and beefiness to a team, plus he has a semi-not-reliable engage tool
Cho is definately off-meta. Nobody plays Cho. But I've played him a lot and know his potential utility, as well as how to play him into every match up possible from Riven to Jayce to Lissandra
Now, when you look at my top lane, you notice that they all benefit from sustain, tankiness, and, above all, relatively safe laning. That's just my style. I play safe. My #1 champion at the moment is Janna. Play your own style. I know people who play Riven/Wukong/Xin Zhao toplane, and don't even bother with Teleport and just play to devour souls. That's cool, as long as you know what's what. The most important thing is to be self-aware of your own limitations.
This has gotten sort of ranty. The important thing to take away from all this is how important consistency is. It doesn't matter how many champions you play if you win more than you lose. Be self-aware and try to look at metrics to see how well you do. For instance, if you're averaging more than 10 deaths per game as Master Yi over 10+ games and are rocking a 40% win rate with him. Maybe you shouldn't play Master Yi.