Work is out by lumens per square foot and you'll get a better idea. 2,000 to 2,400 lumens per sq. ft. is adequate; over 4,000 puts you into solid grow area. Over 5,000 is great.

Most 23w CFLs put out around 1650 lumens. For one square foot, that's three bulbs. You can get away with less when vegging because the plants take up less space. If you keep your plants in 6" (around 1 gallon) pots for a while, you can veg 4 plants with only 3 of the 23w bulbs. But once the plants start to really grow, you're going to need more light. That why veg cabinets can get away with fewer cfls and lumens; they have smaller plants, at least for a while.

Flowering is when all the growth really takes place; you'll need more light then. So the big question is how big do you want or expect your plants to get? In my experience, one decent size closet grow plant takes up around 1.5-1.75 sq. ft. That's 6 to 7,000 lumens to do well...85 to 100 actual watts of CFL per plant. If you keep your plants small, you might be able to get away with a little less.