There's really not much to it. Each management agent is assigned to an Exchange server. As there are 5 default messaging agents, you could add users from 5 different Exchange servers before agent sharing would start. If you're concerned that too many users are being added to the same two agents, you can always limit the number (to 100 or so) by using the registry key MaxUsersPerAgent (HKEY_LOCAL_MACHINE\SOFTWARE\Research In Motion\BlackBerry Enterprise Server\Dispatcher). The default is 500, so this is another variable for agents being spawned (if you have one Exchange server and 800 BlackBerry users, you'd have one agent with 500 users and another agent with 300 users).
Needless to say, as for a defacto document on how it all works, I'm not sure if there is much more than what is listed in the Administration Guide and perhaps the Feature and Technology Overview (it's been a minute since I've read either one). From a health perspective, it may be a good idea to decrease the number of users and utilize all 5 default agents (EVS1 would have 3 agents - 100, 100, and 20; EVS2 would have 2 agents - 100 and 88). If you look into limiting the number of users per agent, it may also be a good idea to increase the number of agents that can be spawned from the default 5 to 10 (they will be spawned on an as-needed basis, so all 10 won't be started if they aren't required) -
NumAgents is your registry key to modify (see linked blog article from the legendary hdawg).