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?
Rembrandt II, Suiché
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen
Gargoyle: Cloudjumper and Toothless
Oh okay. Yeah 20 seems like way too many haha. I guess I'm lucky because my LFS has CPDs. Will CPD's be happy with all males or all females? Or ( I researched them the whole day) with the 1:2 MF ratio?
However, having kept this species in the past I would be hesitant about putting a school of them in with a betta in only a 10 gallon tank. They are known for being a quite skittish and sensitive species, and so they may spend a lot of time hiding, particularly if the betta harasses them. Also, is your tank cycled and able to handle the bioload of a group of fish? These are not bettas to survive very long in poor water quality.
I would definitely not try and put 20 of them into your tank. I think a group between 6-10 would be my limit. They do have to have space to swim as they are quite active fish.