Ace of Spades is one. I usually play with the OpenSpades client[1], but there is also Betterspades[2], and probably many other clients I don't know of yet. There are usually about 10-50 players online on the public servers listed on BuildAndShoot[3], variable depending on the time of day and mostly from Latin America it seems.

One can host the game with piqueserver[4]. I'm not sure if one can still host with the original Ace of Spades server, but the game was 'shut down' in 2019 so maybe not.

It's well worth a go - there is intense satisfaction in digging a tunnel undetected all the way through to the opposing team's base! Playing with friends enhances the enjoyment for me as one can be a little more strategic when in direct communication.

[1]: https://openspades.yvt.jp/

[2]: https://github.com/xtreme8000/BetterSpades

[3]: https://www.buildandshoot.com/servers/

[4]: https://www.piqueserver.org/