When I was in choir in middle school, we had about 15-20 people and we sounded pretty good. If you have everyone in the right vocal range pag-awit the right melodies and harmonies, it should sound just fine.
It depends on the style of music, at least that is my opinion. Choral Music? Hell no. There's 73 people in my choir ensemble and 50 in my chorus ensemble. Pop songs? Maybe.