OpenVPN Access Server is not licensed based on users. Even with the free/unlicensed mode, you should be able to define more than two users.
Instead AS is licensed based on concurrent connections. Without purchasing a license, you are limited to two concurrent connections.
Some advanced features such as fail-over and clustering may also not be available without purchasing a license.
However, the two connection license is built-in forever and there appears to be no intention to remove that in any future version. There is no time limit or expiration.
This web page may help explain it better:
https://openvpn.net/for/free-vpn-access/
Otherwise, if you need an official response (I am not an employee of OpenVPN AS, just a user), try filling out the contact page regarding "Sales & Licensing" here:
https://openvpn.net/contact/
