Reddit's /r/ancientcoins has almost 50,000 members, not that everyone there is a collector, but it indicates a pretty large interest in the hobby (though with the top post only having 1k upvotes the engagement isn't quite there). 50,000 seems really conservative now (though maybe it was a good guess 20 years ago). But communities are rather fragmented, a lot of collectors are on different forums, Facebook groups, maybe a lot of the old guard doesn't even interact with anyone online. Also, this is an English-language forum, there are probably groups in other languages as well too.