You can have as much as you want. It just depends on how much you're willing to spend, what looks good to you, and what works for your setup.
The more light you give your plants, the more demanding they'll be. Meaning, more Co2, more ferts, and more pruning. Otherwise, you'll run into a whole slew of problems, especially with algae.
Buying one (1) 55w Cf. will give you nearly 2 wpg (Low-Med light). A safe way to go for beginners.
But if your willing to spend more time on your tank, then go ahead and buy (2) 55w Cf. But I warn ya, you'll have to keep up with co2, etc., in order to make it with a highlight tank. It's not easy.
HTH
Lissette