The easiest way would be to run a sql query.
Use EnterpriseVaultDirectory
Select count(*)
from exchangemailboxentry
where MbxArchivingState = 1
That will give you your number of enabled users.
Here is a technote for reference:
http://support.veritas.com/docs/309166
How to view a report of disabled, or new user mailboxes from SQL
Details:
On occasion a list or report of new, disabled and enabled users may be needed. This can be done from the Vault Directory SQL server. A SQL query can send the results to text or to file Report Viewer .rpt format..
For disabled mailboxes
SELECT *
FROM ExchangeMailboxEntry
Where MbxArchivingState = '2'
For new mailboxes
SELECT *
FROM ExchangeMailboxEntry
Where MbxArchivingState = '0'
For enabled mailboxes
SELECT *
FROM ExchangeMailboxEntry
Where MbxArchivingState = '1'