Also depends on your space; I grow with the same 125w blue and 250w red and only have the space to grow one plant - although I got nearly 5oz off the one plant (in a 15l pot).
I personally would say given the size my plants have grown, I could get away with 2 plants in 8 liter pots (if I had more width/depth), but I know people who grow lots of plants in 1l pots under 125w CFL to keep them small (kind of micro SoG) - although I imagine this is a ball ache to water.