Hydro setup lights and amount

Im running a 4 x 4 tent with 1 x 600w hps/mh bulb setup in an air cooled reflecter. Was curious to know should i be running 2 bulbs in this soze tent or will 1 give me enough light efficiency ?
Also how many plants could i run in that tent at once doing 4 at the moment with about 6-8 week veg so plants are big thinking of doing more plants with less veg time thoughts ?

A six hundred is right for the space, and you can run 6 plants no prob in that space.

You could do 16 plants in 6.5 litre pots from clone with vey little veg time. 9 plants in 11litre pots with a few weeks veg, 5 plants in 15 litre pots with 5-6 weeks veg time.
Less if you want with bigger pots and more veg time. Its up to you with that space. You could add a light if your ambient temps are low enough or you have enough extraction. There is no need to though. Keep it simple for a few runs and you will know what suits you best soon enough.
GL

A 600 in a 4x4 works great.