Most say 2-3 sq. Ft. Of floor space per bird. but, the more it think about it, thats not a good judge. It really depends on the amount of boxes and perches. after all, the only time pigeons are on the ground is to eat and drink, which is only about 1\2 hour of the entire day. So, do you have box perches? Those are the best, when trying to be able to increase your flock size in a smaller loft. It's really important for each to have their own perch, without having to fight for it. So, I would do a whole wall full of box perches, and you would be able to get 20-24 birds in there, if you are scraping perches at least once a day. As well as boxes and floors.
I have 5×3 loft, that i use for just one pair to breed. But, it has 17 box perches. I would be willing to put 12 birds in there, as long as I kept scraping perches daily. But most other things would say a max max of 7 birds, with 5 being better. Anyways, that's just my opinion. Good luck.