As many as you want (within reason, don't go overboard and swiss-cheese the bracket or anything, you don't want to add so many holes you compromise the integrity of the Hovr), as long as you adhere to the previously explained position of the holes in our article about where to drill the holes in the bracket.
Also, ensure that the holes are not too close to each other and that the pan head screws' heads do not hinder each other from sitting flush on the bracket's wall.