There are currently just over 51,000 games on Steam—but it turns out that, until recently, the client was prone to crashing if you owned even half of them.
An update for Steam's beta client pushed yesterday fixed a possible crash for owners with over 25,000 games in their library. As noted by the r/pcmasterrace user who spotted the patch, this could mean that someone who currently owns such a vast collection noticed the issue and reported it to Valve.
Working out how many people this may even have affected is a little tricky. Steam analytics site Steamladder currently lists three people as having passed this threshold. Their public Steam profiles don't show quite as many, however, as it appears this number omits hidden games. SteamDB has its own chart of users who supposedly own over 25,000 games, using the associated badge to keep track, and reckons there's a good few hundred more. According to their SteamDB page, Steam user Sonix apparently owns a staggering 42,061 titles.
Of course, it's also possible that this was just a small issue discovered by Valve's own testing, making Steam more stable for folks with ludicrous library counts. Steam is only getting bigger, with 10,263 games added in 2020 alone. Collector's libraries are only going to increase in size—and while we can debate who actually owns the most games on the platform, collectors can be fairly sure the client won't buckle under the strain anymore.