i would say three at most... comfortably id say more like two... also, those kinds of cage are exactly the safest or most useful... you cant hang toys from the ceiling without them all sliding around and if it's heavily decorated (as most cages like that are) you keets could get their toes stuck in the points that the wires make causing them to hurt or even rip off a toe... badbadbad...
if thats the one you absolutely want to keep and you are serious about breeding (meaing you know all about all the dangers to the hen and chicks and are ready to keep or find good homes for all the chicks) I would say just get one pair, a male and female. That way they can bond and no one will get left out. NEVER EVER get more females then males if you can help it, they'll fight eachother over the male...