I say go with 5. 4 on the belt and one in your hand/pocket to be used when make ready. This will do just fine for your average match. That is unless you throw a whole mags worth of lead at a piece of steel.
There was a match here in CO that I wanted to shoot but didn't because I would have needed at least three more mags. Each stage had a minimum 60 round count. One stage was sixty pieces of steel. It had 2 Texas stars. Really I wouldn't want to shoot a match like that with anything less than 10 mags.