DSL/Cable for hosting a game is quite a common question. I've ran them on both and I'd have to go with DSL when it comes to hosting. In most cablemodem services, they do things to the line while you are using it. Sure, it doesn't mess up your online gaming experience, or toast your download, but it certainly is noticeable when hosting a game server. When I had DSL, this wasn't a problem. I have a friend who uses business DSL and its basically a T1 for his game server (24 people).
As far as the amount of players is concerned, you can pretty much guarantee 4k/sec for each person. So, you want a 20 person server? 20 * 4k/sec = ~80k/sec of upstream. 80k/second isn't going to be the el cheapo DSL, its going to be expensive DSL ($200+/month). With my 35k/sec upsteam, I can get about 8 people online and works pretty well (only problem is when my ISP tweaks with the line).
Hope that helps.
vash