EDIT 3+: Slight Math Error:
Pure Shield + Fire Affinity + Cover Affinity = 104% Ice Weakness, not 105% as it should be, as 70% * 1.5?
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This bookshelf doesn't have any interaction?
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Given how many potions are already available, and any smart potion user would be using Recycling as well, and then there's all the other ways to get ingredients, a person's favorite potions should be easy to keep from running out, so finding useful gaps to fill with personal mixes are probably small, and therefore 50 does sound like a good number!
EDIT: The most useful idea I have right now, is an Omni-element potion, to best use the equips that give boosts to every element Amp, depending on how strong those Amps are.
Would be crazy to wear every Affinity badge just for that, giving me 210% Amp in everything: Unusually Strange Hat, Sage Cloak, This Thing 3, All the Affinities, and Elemental Fury.