I can't tell you what the west does, but how we do it in this shitty state of NY. Here is my experience the last 3 years shooting some competitions locally. I live about 60 miles from NYC. There is a club in Dutchess County that does monthly USPSA and IDPA matches. I have shot the USPSA matches and no matter the division you are shooting 10 round limit in the magazine. The courses usually run less then 30 rounds from the ones I've shot. The few I did shoot I was able to get by with 4 magazines. For 3 gun I am part of basically an informal private group/club that has been shooting for some years now. It's basically a word of mouth group. Some brings you, you shoot the match and as long as your safe and not a complete ass you get invited back and put on the email list for matches. A couple guys belong to clubs that then in turn allow us to have the ranges for the day for our matches. When I started 3 gun (3 years ago) we were still running 30rnd mags. There was still the exception if they were pre 1994 they were legal. So we all had "pre ban" mags. We would have some good rifle stages. Now with this f-ing SAFE act all that is out the window. We now are running the 10 round mags (as you can see in my profile pic from last year). Typically the rifle stages are less then 40 rounds. So yes there are mag changes. You get familiar with the mag release buttons on you firearms.
With the SAFE Act the part of the 7 rounds in the magazine law was found to be unconstitutional and thrown out, but before this there was an exception in the law that as long as it was a competition or on the range you could have 10 in the magazine.
All mag changes are on the clock. Stages are not generally not to intensive with round counts.
Any other questions let me know. I have no idea what the rest of the state is doing. I actually haven't shot a match since the beginning of April. I hope to get to the June 3 gun match. Generally it is 1 match a month and unfortuantly the days the guys could get their local club for our 3 gun is the same as USPSA. Typically USPSA starts March and ends November and out 3 gun starts April and ends September.
I wish we had it like some of the other states with some type of match every week.