On some NetScaler VPX you might find the default count for "Maximum Access Gateway Users Allowed" as 15.
Adding an additional SSL_VPN/Universal license for 5 users should change the "Maximum Access Gateway Users Allowed" to 20, however the maximum users becomes 10.
This is a known issue where the default number of "Maximum Access Gateway Users Allowed" is shown as 15 instead of 5. This has been fixed and the latest NetScaler firmware offers 5 "Maximum Access Gateway Users Allowed" instead of 15.