Dota Underlords' beta went live yesterday and, at the time of writing, it already has more than 170,000 people playing it concurrently, according to SteamDB. This is just based on Steam numbers and does not include all the Android and iOS players.
Dota Autochess quickly became one of the most talked about mods, even beyond the Dota 2 community, echoing the original Defense of the Ancient's rise to prominence. There are already a few new versions in development, though Valve's been pretty quick in getting Underlords out. It's not surprising, then, to see so many people flock to it.
As GitHyp notes, this is quite a bit more than Valve's last game enjoyed. Artifact had around 60,000 concurrent players on its best day, but they quickly stopped returning. Valve's gone back to the drawing board and isn't going to be talking about it until it's figured out some solutions to the many criticisms. In the meantime, Underlords is in the spotlight.
One of the big differences is that Underlords is free. It's also still in Early Access, however, where it's expected to say for a few months. Throughout that time, expect new features like more Underlords, ranked matchmaking, tournament tools and a battle pass to be introduced.