The last time I used openvpn, it was version 1.x and only supported a single connection per running process. So, if you had a server that you wanted to support multiple clients connecting, you had to… have multiple ports open to the outside world (unless you did something VERY fancy), and had to have as many openvpn processes open and listening for connections, as you had clients you expected to connect. It wasn’t a pretty setup unless you had a small number (1-5) that you expected to connect. Fortunately that has changed with the 2.0 series of openvpn and it’s really matured as a vpn solution.