It would depend on the strain, as in how thick and bushy and wide they would get and if you may increase that by topping. I would say two good sized plants, not whoppers .. just good sized plants, would max you out. A 250-watt HID light should cover an area 3ft. X 3ft., so it would not be lack of light but instead lack of overall area/size to fit in more plants without cramming them in to tightly.
 
That of course means you will want to use feminized beans or grow from clones because you will not have the room to grow enough plants that after separating males you will be sure to have two, or maybe the three, females plants you hope for.