https://w3techs.com/technologies/overview/content_management/all https://www.whoishostingthis.com/compare/wordpress/stats/ The first site which comes to mind for these things is W3Tech as they have a fairly long reputation of tracking these things, although they don't give you precise numbers. Luckily, we know how many sites Wordpress has, so we can calculate backwards off that. Wordpress has 75 million sites and 33% of all sites. vBulletin has x sites and 0.1% of all sites. Let's start by dividing 75 million by 33. That's about 2.2 million per percent. Divide that by ten and that's 220k sites. These statistics sites tend to give varying results however, perhaps some crawlers can't find all the sites while others can find other pockets of sites. And they don't need to have millions of paying customers, if you know what I mean, although who knows, I just follow whatever statistics seem reliable. Edit: I give up on calculating. My brain is half-asleep. Anyway. I think that methodology could get some results. I'm going to bed, good night.