depends on how big your plant it, tall, thin ,... thick??? bushy??
hum.. 2x2x4
try 3-4 or 5. 23w-26w cfl bulbs 2600k (lightcolor) that's about 6ooo lumen for a 2x2x4 that should be fine, especially if you have reflective walls, if you feel its not enough light add more lights..