It depends on the pot size your planning to use if your using 1 gal. then youd only get 3 or 4. And if you plan on going from clone with no veg cycle remember that the transition is not instantanious, it take a couple of weeks. In this time your girls will stretch. Ive seen clones wind up 1 1/2- 2 1/2 feet tall with ease under a good light. If height is an ussue for building you box bigger then go wider. And attempt to do almost like a s.c.r.o.g and lean them over to allow only the buds to pennitrate to the light.
