Player numbers aren't that great daily on steam (considering most owners bought the epic version anyway), but all of the developer tracks plus most of the community tracks have leaderboards in the tens of thousands, even the hundreds of thousands for the seasonal track releases. There's also a daily community highlight where players can race and compete on a community track hand-picked by Nadeo staff, so that really bolsters numbers on community tracks.

Multiplayer is kinda dead compared to the older games, but I also don't think a lot of players play the game for the multiplayer anyway. There are even community playlists of tracks that you can play in singleplayer that go up to I think 50 tracks in size. So you can just pick one of those and play through a bunch without ever having to connect to a community server.