for the larger installs, how many users are you able to run on a box?

how many bes server instances per box?

we run to logical servers per box, 425 users per bes server instance.

You are limited to mapi connections which they say are 500. So 500 per BES instance. We create a new bes when we reach 400 to give us some "head room" You can have up to four instance of BES on each physical server.