Get a count of mailboxes by Exchange 2010 database

get-mailbox -Server MBXServer-01 |group database

Results:

Count Name          Group
----- ----          -----   
   60 mbxdb-8      { <username>, <username>, ...
   56 mbxdb-3      { <username>, <username>, ...
   53 mbxdb-4      { <username>, <username>, ...
   31 mbxdb-25     { <username>, <username>, ...
   32 mbxdb-22     { <username>, <username>, ...
   31 mbxdb-21     { <username>, <username>, ...
   16 mbxdb-12     { <username>, <username>, ...
   19 mbxdb-11     { <username>, <username>, ...
   24 mbxdb-7      { <username>, <username>, ...