:) How good is your filter.....Thats the question. As many as you can fit without them hurting or killing eachother. If you have a great protein skimmer or filter or w/e, then your fine. But if your filter sucks.......invest in the filter first......keep your water prestine and load her up to the brim with corals.
As long as you have the equipment...you should be a okay. Make sure you compensate for sweeper tentacles....My galaxy corals will extend around 3-4 inches at night. I would stock it up as much as you feel necessary.