10s of thousands.. hahahah ok!
For whatever it's worth, both JNPG and TGC 2019 show 20,000 - 50,000 owners on Steamspy. As for recent, current user numbers; TGC 2019 showed a peak of 335 concurrent users yesterday, while JNPG showed 88. I don't know how typical this is, but that's yesterday's sample.
I was the president of a Society for the PC version, after the release of TGC 2, called DSP - Digital Sportspage (most of the members knew each other from the days when the webzine, Digital Sportspage, was in publication with its own forum). It peaked at 16 members, but the participation died off to 3 or 4 a week after about four months. This user base has been slowly converting to the current gen consoles since their advent, so most of those guys went to the XBox One, especially after the XBox One X was released and was outperforming a lot of their gaming PCs. They're all playing in the XBox One version of Digital Sportspage Society now.
I do know that there are at least a half dozen fairly active, independent PC Societies now, and of course you have the massive TGC Tours organization that integrates all three platforms on on leaderboard for the various levels of Societies they host. There are probably close to 10,000 members in TGC Tours, across all three platforms. I play in the Challenge Circuit, which is the third tier in their hierarchy, and I'm in the bottom class, "D" of that class. It's pretty intense, but I have to confess that my skills at the game are declining as I go further into my Autumn years, but I enjoy posting a 66 now, whereas a year ago I would have been disappointing in it.
JNPG still gets an amazing amount of support from the designers in this community, which I think helps it keep a pulse. I might play it a couple of rounds a month when I hear of a new course, or here lately, want to check out what's being done with 3D grass. I don't know how much longer it can go before it's not worth PP's time and money to keep in service. It's already gone further than I thought it would, but that's a testament to the gifted designers who still have a passion for it.