Ten rounds per mag is law here. I drop mags and reload at specific intervals (when moving) so that time is saved. It doesn't pay to run the gun empty in the middle of shooting. Knowing when to mag change is part of the strategy.

Do I count the bullets mid-stage? Generally one does not. Over time a shooter gets a feel for it and tends to know. Same as above, running the gun dry is dumb, so one will often change with rounds still in the mag to avoid it. Depends on the situation in the field or a competitive shoot.

Part of the strategy in a shooting stage is to know when those mag changes will happen, at what spot on the course. This assumes a perfect run with no misses as that will mess up the round count. So one counts the rounds required, programs the mag changes then just sticks to that if all goes well.

Yesterday I dropped a mag on one stage after only four rounds so that I could proceed through the next little bit with a full mag. Changing mags when moving saves more time than doing it whilst trying shoot stuff.