Popular L4D2 modder and map maker Sky has written a great tutorial for configuring your Left 4 Dead 2 server to support more than 8 players. With his instructions you can have up to 18 or 24 players on a single L4D2 server! That’s a lot of madness.
Naturally, we have a test server up here with 18 players! Hosted Game Servers really shows its power by hosting this kind of madness with no sweat.
Check out the detailed how to which includes server configs and mod installs on the Hosted Game Servers forums here: http://forum.hostedgameservers.com/viewtopic.php?f=28&t=82&start=0