I disagree with the 1 trick pony. Perhaps for some really unique champions it makes sense, but I think it's very possible and better to just be a very selective player.
Benefits of 1 trick pony:
- Lots of practice on the champion
Cons:
- Champion can get banned / picked
- Have to pick it in to counters
- Could become very weak due to meta shifts
- Have to pick it in to teams that don't want it
- Can get bored
Benefits of selective champion choices:
- Lots of practice on ~5 champions
- Have options to deal with counterpicks
- Have options to go with any team comp
- Keeps things fresh
- Better overall understanding of the game
Cons:
- Perhaps not huge mastery of any individual champion
I think the average person has a skill-cap imposed on them by their gameplaying abilities (reaction speed, awareness, quick decision making, etc...) which are unchanged by playing only 1 champion. I feel like I've peaked on most of my main champions from an individual champion standpoint - my room to grow is now in the game as a whole or on new champions. Some few players may have the potential to get insanely good on an individual champion (think BoxBox on Riven) but these people are NOT the typical league player and odds are you won't reach that level and go challenger by being a one-trick.
So take some time and come up with a champion pool. Pick your two main roles. Pick your best champions in each. Try to find champions with similar playstyles but fulfill different roles. E.g. Corki and Quinn are both ADCs that depend heavily on their abilities and have access to a major MS buff, however one does magic damage and one physical, one is for poke and the other burst, etc... (so they work with different comps).
Likewise Zed and Ekko are both melee mid lane assassins that have huge outplay potential. However Zed is AD and Ekko is AP, which allows you to bring different offerings to your team.
So say you want to main ADC and Mid. There's 4 champions that offer a huge variety of gameplay options, you can focus on getting really good on all 4 and play whoever fits your team best and/or is strongest in the meta at the time. So when Zed gets nerfed you still have a strong Ekko to play, etc...