Yep, there's really no limit to the number of corals you can have. However, you should be careful with what you put in there, especially as it's a small tank. A large leather, for example, could negatively affect LPS you have. Some corals just get too big for a small tank, so think about that when you're choosing, too. Also, bear in mind that while you can mix soft corals and LPS, mixing SPS with corals that prefer a nutrient-rich tank will put one set of corals at a disadvantage.