Its total weapons, not by type.
20 is the max you can put in an arc w/o FC tonnage being added.
It doesn't matter if its 20 AMS, 20 LPL, 20 Heavy Naval Gauss Cannon, or a Mix of them all.
AMS counts towards limits, BUT, does not count for gunners. So the crew is less but its still part of the 20 weapon soft-cap.