Three firepoints max per turret I think. I have no idea what bumping that limit would entail.
You can put up to 10 firepoints per turret, but only three weapons (four in case of missile turret, there is also possiblity of making hybrid gun-missile turret with 7 weapons on it).
There are a plenty of flags and .tbl entries that can make them fire in cycle, in bursts, simultanously, or in many other ways.
Plus we have "same turret clooldown" which ensures maximal rate of fire regardless of skill level and AI class.