Many carried 2 or even 3 bandoleers. Also, magazine pouches on web gear/ALICE . But all were 20 round. In my day...Unfortunately, we were also ordered to load only 18 in each magazine as the springs were prone to "breakage."
Downloading a milspec magazine..
Breaking springs may have been an issue with the early 20 rounders, but today, the reason for 18/28 round loading is because it is very difficult to insert a fully loaded 30 rounder into the rifle with the bolt-carrier forward (admin reload or tactical top-up). The mags were built to hold their capacity without much extra wiggle room.
NOT the case with any capacity MagPull P-Mag! Fill it to the rim!!!
KYPD