Few MS Exchange 2010 Commands

Getting the total count on the mailbox is very easy using Exchange Management Shell Commands and below are few useful examples
How to get the Mailbox Count in a Database

(Get-Mailbox -ResultSize Unlimited -Database "Database Name").count

How to get the Mailbox count in a Server

(Get-Mailbox -ResultSize Unlimited -Server "Server Name").count

How to get the Mailbox Count from the whole organization

(Get-Mailbox -ResultSize Unlimited ).count

Ok, I got the total count, now are you searching for option on how to get the room mailbox count?

(Get-Mailbox -ResultSize Unlimited -filter (RecipientTypeDetails -Eq "RoomMailbox")).count

How to calculate the Mailbox count which includes the disabled mailboxes in the database

(Get-MailboxStatistics -Database "Database Name").count

Hope its useful for you. Leave your comments if any details required.