I have only 30 round mags.  But the devious North Texas stage designers are working in AR stages where a 30 round mag just gets in the way and forces you to take a less optimal position.  Things like laying on a slanted platform with gaps in the wood slats and shooting out to 300-500 yards, from inside a car or under a car.  I'm coming up to another one of these devious stages in a week.  You guys that have experimented with 10 and 20 round AR mags, which one did you find was the way to go.  If I got the 20rnds, I'd probably get 2.  If I got the 10rnds, I'd probably get 3.  If the stage has a mixture of positions/shots, I'd probably start off with a 30rnd and mag a mag change to 10/20rnds before going into cramp quarters.

when I shot the last four of the North Carolina Tactical Matches, you expected Kyle Lamb to incorporate unusual shooting positions... we shot under Checker Marathons at one, and ran through a couple more... so I added a green follower to a couple of 20 round Colt magazines with a new springs...since the demise of his matches, one is in my equipment box, but really haven't needed to use it since... at Area 6 this year, several courses were for 32 or more rifle rounds... ordered a Nordic Component 19round extension for my PMags

I'd go with the 20s.  I usually bring one just in case.  The 20rd mags are shorter than the pistol grip, so if you need to get any lower, the pistol grip is going to be in your way, not the mag.  Having something shorter like a 10 won't help at that point.

I always bring a couple of 20's with me for those odd shots or shots where you are in prone.  It would be exceedingly odd where a 10 round magazine would do any better than a 20.

I have a couple 20s I use for weird positions or when a mandatory reload off the belt is required. As I reload a 20 round mag faster than the 30 as I can index it just like a pistol mag

I take 3 20 rounders to the NTMG matches.  I keep one on my belt as a "back-up".  The other two always get used on the prone stages.  I use Magpuls and they seem to work well.

I picked up 2 smoke/clear Lancer 20 round mags and used them at the last NTMG match…good decision.  There was a prone stage where every shot was prone…no problem with the mag getting in the way and I flew through the stage.

Have you considered 15 rounders ? I like them because they are ( size wise ) in between the flush 10rd and standard 20rd.

https://www.wideners.com/itemdetail.cfm?item_id=100001593&dir=625|1056|1057

They also carry ones with a grey Magpul follower.

I use the Magpul 20 rounders.