Cube Engine 1 and 2 touted as the smallest (in terms of file size) FPS Multiplayer game. It's also free. Unknown if the community is still booming as I have not played it in a long time. Eh while we're at it there's also Warsow but this one I know the community isn't as great as before. Source: about 1 year ago

I'd you want something a little more advanced but straight forward I highly suggest Warsow. It's a free first person arena shooter that plays very much like Quake. Source: about 2 years ago

Warsow is also quite good and very close to late 90's shooter's idea of multiplayer https://warsow.net/. - Source: Hacker News / about 3 years ago

Playing games is part of who we are as humans. Ditto for making games. From Nine Men Morris to Shattered Pixel Dungeon, Mindustry, and Warsow, people have been making and playing games for free. We don't need a games industry in order to have games. Source: about 3 years ago

If you still want an estimate you can check on Steam Spy. Source: about 1 year ago

I think Valve will shut it down once it learns about the monetization. AFAIK that is what happened to https://steamspy.com/ when they introduced paid subscription tier. - Source: Hacker News / over 1 year ago

If you still want to see numbers, you can try with Steam Spy with VN available on steam. Most of the developers say that those numbers are false. Source: over 1 year ago

Research popular game genres, for example on https://steamspy.com, and find a genre that currently sells well. Source: almost 2 years ago

Steam Spy - must have tool for anyone who wants to get some data about Steam games. Source: almost 2 years ago