There is not a way to get a count of the number of entries in the GAL through the address book interface. That being said, the entries in the GAL do not necessarily correlate 1:1 with the number or users in an organization. Moreover, the Offline Address Book (OAB) that Outlook uses while you are offline does not necessarily match up 1:1 with the GAL, though that is the default configuration.

I wouldn't depend on the GAL/OAB for anything other than presenting a list of addresses you can send emails to, and not a complete list necessarily.

My guess is that the OAB has a programatic inteface that you could script a count through, but I haven't gone looking for one.