hi, if its a brand new tank dont add any more fish at the moment, as it probably wont be cycled yet, but in time u may be able to put a few more fish in, depend on how many plants, filtration, frequency of waterchanges etc.
have a good search on the net for info, theres heaps available
your LFS should be able to test your water 4 u and tell u whether your tank has cycled, its usually about a month, though the general rule is add fish slowly. many of us learn this the hard way
anyway good luck with your new tank, and welcome to the forums