When I was messing around with this, I tried Arredondo +2 bases without the finger insert. I had some of the regular inserts for STI mags. The finger insert is used in the Para 9/38 tubes & is not needed in 40 mags. The fingers are delicate & deform easily. With thoses base/inserts, & Arredondo springs & followers, I got 19 if I recall correctly, but the follower would usually get stuck in the tube.
If you are serious about running this gun without mag issues, just get new STI mags from anywhere, change to Grams guts, Dawson or TTI bases, be happy with 20, & go shoot. For the most part Arredondo mag parts are pure crap & will disappoint you.