Number of DB's in a DAG

Question

I'm using the requirements calculator and having trouble understanding the recommendations it's coming up with in a certain scenario. At first I have put in that there will be a total of 4 Mailbox servers (2 in each DC). For "Total number of HA DB Copy
instances within DAG" I have put 4 (thinking there would be 1 active DB on each server). I then put 2 for the "Number of HA DB Copy Instances in SDC" so there would be 2 active in PDC and 2 active in SDC. We have 2,000 users, so there should
be 500 in each active DB. 
This results in the Role Requirements showing what I expect - 2 active DB's in each DC, a total of 4 DB's with 500 users each. 
What I don't understand is what happens when I switch the number of mailbox servers to 8 total (4 in each DC). I change the "Total number of HA DB Copy Instances" to 8 (one active DB per server), with the number in the SDC to 4. I would expect this
to result in a recommendation of 8 active DB's with 250 users each. Instead the calculator is suggesting there be 24 databases with only 83 users each. Each server is stated to have 3 active DB's instead of the 1 I was expecting. 
Can anyone shed some light onto why this might be happening?

Answer 1

Hi MRCUR,
According to your description, I understand that you want to add more DAG members and active DB to reduce loss when mailbox server failed.
I agree with Ed, we have to balance cost, availability, and performance when we design a DAG. We can enable more active DB under each member to balance the amount of member and DB.
More details about Exchange server performance, please refer to Example of Sizing for an Exchange 2010 Multiple-Role Scenario section under
Understanding Multiple Server Role Configurations in Capacity Planning:
http://technet.microsoft.com/en-us/library/dd298121(v=exchg.141).aspx
more details about Database availability groups, for your reference:
http://technet.microsoft.com/en-us/library/dd979799(v=exchg.150).aspx#DAGforSiteResilience
Best Regards,
Allen Wang