How big are higher elo champ pools?
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What do you mean "high elo"?
I peaked diamond 4 and this is my champion pool: 
Champ pool has nothing to do with ELO. There are players in challenger who literally only play 1-2 champions. Also there are players in bronze who play 30 champions. Its all about players personal preference.
i'm not high elo but I have heard that playing 1-3 champions over 2 roles is generally what you want