We have players that play in the Fire Direction Center role, who take call for fires from players who are playing as Forward Observers.

This is all done via realistic radio communications handled by the ACRE mod (which I am the lead developer on as well... a radio and artillery, when combined, the deadliest weapon on the battlefield! :p).

We often have 70 players in game at once at united operations.

I don't know if you are familiar with US artillery doctrine vs. Russian, but in the US its a lot lower level, a platoon commander can directly call for fire from corp level assets (MLRS or back in the day M110s) in some situations, where in the Russian military that would be a much higher level call. There are also much lower level artillery assets, battalion and divisional assets that are directly under control of their associated unit commanders.

I'd advise reading Fires/Field Artillery Magazine/Field Artillery Journal, which are all available, back issues all teh way back to the turn of the 20th century on the Ft. Sill website.