I've read somewhere that Openarena (and Quake 3 in general) uses about 5-10 Kbps per player. However, these numbers may vary depending on server configuration. Look up Quake 3 articles that talk about network usage.
I have seen on the net calculations of Q3A server bandwidth usage (unfortunately, I cannot recall where). The figures they come up with were much higher. On a 400 kbit/s upstream connection you should not really allow to connect more than 4-5 players.