There is no set rule. It solely depends on what species of fish you get. For instance for a Tang I would say '0'.
There are many saltwater fish that just can't live in a 30gal tank. But say for instance you were talking about green chromis or something similar I would say you could have 3-5.
but say you were talking about a mandarin... then I would say none.
as a rule of thumb there is no rule of thumb, you MUST research each animal before you purhcase it, get several different opinions. (don't rely solely on your LFS, remeber they really only want to sell the fish they have) and make an educated decision on each fish you buy for your tank.