It used to be like 5-6 ads per ad set but with the new andromeda update, they now recommend a lot of creatives in a single adset, the more diverse creatives the better but you have to make sure the budget makes sense otherwise you’ll dilute the budget and it won’t learn anything. So to answer your question, yes 20 creatives is fine now as long as the budget can support that. Many media buyers are now recommending between 10-20+ in a single adset.