please read this:
http://openvpn.net/index.php/open-source/documentation/howto.html#policy
note that it was written before topology subnet existed, so your ifconfig-push would instead look like ifconfig-push <IP> <SUBNET>
however, you in your hypothetical setup you would run into a different problem.
By the time you run out of IPs in a /24 with topology subnet, you have already started having so many users that your server explodes.
ok maybe it doesnt explode, but openvpn chokes and has a painful death around 200 users... this is because of the monolithic way it was coded in. This will not change until version 3, which will not happen any time soon.
you will need more openvpn services to handle that quantity of users.
note that each instance of openvpn will only run on a single cpu core, so you can run multiple instances on a multi core system just fine.
hope that helps
