Maybe something stupid, but in Active Directory Users and Computers, you can also do this:
Menu --> View --> Customize... --> "Customize View" : MMC : select also "Description bar".
Now, on top of each detail pane, automatically the total number of objects is displayed.
Attention: the count includes the total of *all* displayed objects inside the selected OU in the left pane.
So it could also be a mixture of users, groups, computers and sub-OUs.
Just for the record...