OK... here's the deal... a BES server will support up to 2000 users, in theory... which is all good for a Domino server, which uses NRPC, Notes RPC, I think, it's been a while... to communicate between Notes and BES...
Communication between Exchange and BES on the other hand uses MAPI... MAPI has a little limitation on the number of concurrent connections per profile... of 500. So to maximize the BES services, you can have 4 instances running, each one with 500 MAPI connections giving you your total of 2000 users.
I hope this eplains it.
cd.