Point of diminished returns, just like silencer baffles.
More doesn't necessarily mean better, it just means more.

I originally installed a 6 port (3 pair) brake on my 308.
After a couple of range visits, noticed heavy carbon build up on first 2 pair, but just about nothing on the last pair.
Pulled it, put in lathe, and cut the last pair off.
Next range visit the recoil had not changed, which told me that the first 2 pair were doing all the work.