My server is hosted on an OC48 network. I can host as many as I want but pay for 14 clients with 2 private clients for a total of 16. That's with each client getting 10000bps. And NO lag or ping spikes.
Use this to figure out what you can host on your box.
Go to TOOLS
Go to SPEED TEST
Run the test. See what it says as your upload speed.
If it says your upload speed is 128K, that works out to about 16kbps actual upload speed. That'd be 4 players at 4000bps or 2 players at 8000bps.
With 4000bps you'll have some problems though.
So for just the avg, everyday, I'll put up a server for us to play on, type of game, use this equation.
Total Upstream Bandwith / Max BPS per Client = Number of Players
It'd help if I knew what your upstream capabilities were and I could tell you what your max players should be. However I also need to know your system specs. So your system can handle the information coming in from the clients.