As always, you will get lots of opinions here. I always use the two square feet of space per bird, assuming at least a 6 foot ceiling. Some will go more, some less but one thing I think we will all agree on is that overcrowding is probably the single worst thing you can do with pigeons. Disease, unrest, health issues, etc. etc. are all more prevelant in a crowded loft.
In a loft your size, I would not keep more than 12 pigeons in it. Remember, that includes any babies that will inevitably come along unless you have a way of separating the sexes.