Not sure, but if I had to guess, there are probably more IPSC shooters vs USPSA.
USPSA is only shot in the US and US territories. IPSC is shot all around the world in Europe, Eastern Europe, SE Asia, Africa, Australia, South America….
A few years back there was a push in the the Philippines to break away from IPSC and adopt USPSA rules, but I don’t know what became of that.
I do remember hearing some interesting stats about Russian a few years ago: The membership in the Russian Region of IPSC was way higher than USPSA membership, but the number of active competitors wasn’t very high. The speculation behind that was you had to be a member of a shooting competition organization to own a handgun, and IPSC qualifies. So a lot of people joined IPSC to be able to purchase their handgun, shot a match, then went on with their life.