Four, if you ask me. You start with just one turned on and at about two weeks, turn on the second, then the third and so on as your plant grows. Try doing a search for DIY CFL things for some ideas on how to get them set up in there. Getting it right before you start is better than messing around with them when your plant(s) are growing already- you tend to be too stressed and rushed then, and could mess up your plants.