This VBScript uses native SQL connectivity via ADO to query your BlackBerry Enterprise Server configuration database for the current number of users. Since the license total is not stored in the database you have to set it as a variable in the script. It then compare the two numbers and if your defined threshold is exceeded, it will email you a notification. Just schedule the script to run daily. Download a zipped version here, or copy below.

'BES license usage monitor.  Set current license total, alert threshold, And
'other variables to be notified when the license usage exceeds the threshold.
Dim strSQLServer, strBESDB, iCurrentLicenseTotal, iLicenseThreshold
Dim strMailRecipients, strMailSender, strMailSubject, strMailServer
'*************************************
'------Configurable Variables---------
iCurrentLicenseTotal = 1000 'Total number of licenses in License Management
iLicenseThreshold = 20 'Free licenses threshold for notification
strSQLServer = "ServerName" 'NetBIOS name of SQL Server
strBESDB = "DatabaseName" 'Database name in SQL
'------Notification configuration-----
strMailRecipients = "user1@domain.com,user2@domain.com" 'Command-separated list of addresses
strMailSender = "BESLicenseCounter@domain.com" 'From address
strMailSubject = "BES license usage at critical level" 'Subject of notification message
strMailServer = "smtpserver.domain.com" 'FQDN of smarthost
'*************************************

Dim iCurrentUserTotal, iAvailableLicenses, strSQLQuery, strSQLConn
Dim objSQLConn, objSQLRecordSet, objSQLExec
strSQLConn = "Provider=SQLOLEDB.1;Integrated Security=SSPI;Data Source=" & _
	strSQLServer & "; Initial Catalog=" & strBESDB
strSQLQuery = "Select Count(*) As Total_Users From UserStats"
Set objSQLConn = CreateObject("ADODB.Connection")
Set objSQLRecordSet = CreateObject("ADODB.Recordset")
objSQLConn.Open = strSQLConn
objSQLRecordSet.Open strSQLQuery, objSQLConn
Set objSQLExec = objSQLRecordSet
While Not objSQLExec.EOF
	iCurrentUserTotal = objSQLExec.Fields("Total_Users").Value
	objSQLExec.MoveNext
Wend
iAvailableLicenses = iCurrentLicenseTotal - iCurrentUserTotal
If iAvailableLicenses < iLicenseThreshold Then
	Dim strMailBody
	strMailBody = iCurrentUserTotal & " of " & iCurrentLicenseTotal & " licenses are in use."
	strMailBody = strMailBody & "  It is time to order more licenses."
	SendMail
End If

Sub SendMail
	Set objEmail = CreateObject("CDO.Message")
	objEmail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
	objEmail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserver") = strMailServer
	objEmail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
	objEmail.Configuration.Fields.Update
	objEmail.From = strMailSender
	objEmail.To = strMailRecipients
	objEmail.Subject = strMailSubject
	objEmail.HTMLBody = strMailBody
	objEmail.Send
End Sub