You could easily have 4-5 depending on thier adult size in a 2x3 pen. My breeding pens are 2x2x2 perfect square pens, the birds have all the room in the world. Only problem is you gotta make sure that the hens you put in the roo are on the same hormonal level. What I mean by that is if the roo wants to mate and the hens don't yet, the roo can really hurt the hens in that sort of space. However, as long as both either dont want to mate, or both do want to mate, the only issues you should have are bald heads on hens or ruffed up to bare backs. This is normal in a pen situation. In a more wild situation, it may not be as normal or acceptable. I'm really considering taking a few birds and ground penning them and not selling or eating thier offspring to see the differences in temperments and if there may be any broodiness in doing so.
<-----the last sentence has nothing to do with your post just thinking out loud here
pen size doesn't always dictate how many will survive in a pen together or tolerate each other..Somtimes its acceptable to them and others times not meant to be.
my mature bobwhites(4-5 year olds) don't tolerate extra hens or cocks in there pens. they are pair bonded. Each pair have a 3x3 or 3x4 pen, real cozy and plenty of room for pine branches and small roosts in for them, plus each pen gets a small 12"x12" sand box to dust in..
now my younger ones i can sometimes get away with colony breeding (3x8 pen) within the first and maybe the second year, but not always and normally within there second year its old news and the battles will start, it is quite brutal if not caught in time... fights will break out between males and males, females and females, that happens in wild as well, the only difference is in pens they cannot get away from each other., breeding or not bobwhite will fight.
I think the OP was talking about coturnix which dont do well in just pairs (normally). As the roo over breeds, the roo in coturnix mate just like the roo in chickens do.. (which is fitting since they're closely related).
Yeah I avoid answering too many bobwhite posts because they are pretty different. I wish my birds did pair up better because I'd happily keep more roos and play around with genetics a lot better.