I found that my 12x24 'plate' holds 16 chicks for long enough to feather out.I found the top got too soiled at least once a day. I just hatched 15 chicks a couple days ago so suspect I may be using both of my brooder plates eventually here.
My tolerance for 'soiled' might be different than yours

I do scrape most of it off every few days, the cardboard and underneath.

