Yeah I wouldn't keep 20 of them, 15 is fine. You still want room for them to be able to swim haha.
Also take into account, I don't know about your area but Celestial Pearl Danios are not found around here and you'd have to order them from online. Now if you had planned to do that, then awesome! They are great fish and worth their buck. Not much is known about them yet as they are still realtively new so I definitely suggest doing plenty of research on them
Also keep in mind you need about 1 Male to 2 Females ideally, of course that's not fact but what I think I remember from my research lol so that will definitely have to be checked, but males will spar time to time as they show off for the ladies. So you just don't want too many males all sparring with each other all the time, you know?