you are obviously missing all of the target bans aimed at players hence why you've seen Shen/Ali a fair amount
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I think Riot should continue to use their own judgement on this matter and NOT use the size of the champion pool to completely dictate the number of bans. Sure, they should to look at, but they should also look at more in-depth statistics like how many champions have less than 2 counters, how many have globals, etc.
The ban size is static while the champion pool keeps growing. I think maybe the game isn't seeing a huge impact from this because Riot is getting better at creating new champions and balancing old ones.