Reliably, you can probably only get 9 or 10 players on your server (not including ones on your internal network) The general rule is 5KB/s per player. There really isn't much you can do about optimizing since your bandwidth is the bottleneck. The only thing I can think of is not let players zone
Then you could probably squeeze 15 or more on, though the moment one person camps/zones LD across the board.