Essentially, this would be a number found on the main menu or in some other place that would display the total number of online users playing Tower Unite at that moment, across all servers.
This would be useful because any player would be able to see the total number of players online and thus gauge how popular the game is at that moment, and also to gain a better idea of the many people available to interact with. This is mainly to make the game feel less empty, because players may mistake the playercount of the official server for the total number of online users, which is incorrect.
My apologies if this was already planned. I don’t remember seeing this mentioned anywhere…