im not really sure.. but im also interested in this question as well as i will be purchasing a 250 hps... the one i was looking at had a 28,500 lumen output... and with 3000 lumens per square foot being a good target.. i'd have to say that as many plants as you can fit in like a 2 1/2x 2 1/2 foot square is as many as you can grow...and it also varies greatly on how much reflectivity you have on the walls..... but thats just a guess
good luck
~Notorious