At least double the player count that we could support in M&B: Warband - Napoleonic Wars.
Are you talking about the 300 players that you said on announcement which I assume you stress tested a server and figured out that a GOOD server could support that many or the 200 that taleworlds forced FSE to put into affect.
Or maybe compromise and double the 250 player servers I see sometimes from big regimental events like the 91st and 92nd Conquest events and that we had in Mount & musket servers
either way I would prefer 500 or 600 player servers over 400 any day
EDIT. Nvm found this on the BCoF steam group
● Massive multiplayer battles with 500+ players.