You want a minimum of about 3,000 lumens per sf for proper lighting. The sun puts out about 10,000 lumens per sf, you want to stay somewhere under this limit as too much light can burn a plant. Take what your lamp put out in lumens, then see how much area your light will cover and then determine how many plants will fit in that space.
My 1,000w hps lamps put out about 140,000 lumens when new. I then divide 140,000 by 3,000 and find that this lamp will cover an area of about 47 sf at the minimum of 3,000 lumens per sf. I would call this the maximum area I'd consider lighting with only one lamp.
My grow space is 50 sf and I use 3- 1,000w hps lamps. That's 420,000 lumens in 50 sf or 8,400 lumens per sf, that's a bunch of light. I've had some light bleaching due to my rather high lumen/sf ratio. But this also produces intense buds with very tight growth. I pulled 4 1/2 lbs from 9 plants this last grow.
I'd recommend you go with at least 5,000 lumens per sf.