You can start with one CFL per plant if you're on a really tight budget right now, but you'll need to add more as the plant gets larger. And it doesn't really have to do with the watts of the CFL, but the lumens it produces. Look on the package of a CFL and it will tell you how many lumens it produces. You should have at least 10,000 lumens when you go into flowering. I use 2 15 watt CFLs for side lighting only, and they each produce 1000 lumens. They cost me about $4 each at Lowe's.