You should be able to keep 10 or so in there but they need perches. If you want the birds to be able to return so you can re use them, you will need a trap door too, unless you plan on leaving the main door open for them to come back in. Oh, and not sure where you live but it probably doesn't matter, get ready to deal with a snake or two every year. We love having pigeons, really enjoy watching them fly. I have a 4x4x8' tall coop and 15 is the max for me, I like to keep it at about 10-12. I have 14 right now but it's training season, I'm sitting perfect.
Haven't had it in years but never spit any out.