*Theoretically*, you could host a small MP game on a 56K modem... but reality requires a bit more. As a rule of thumb in my experience, I'd say go with this formula: Minimum should be 32K per player, so let's say you have 4 players total, that would be 4x32K=128K. For 8 players this means 8x32K=256K and so forth. (BTW - all values refer to upload speed.)
But it also depends on the kind of MP game (TvT, Co-Op mission with many enemies etc.) and the map. To more comfortably host demanding MP games/maps, you should at least double those values to min. 64K per player (e.g. min. 256K for 4 players). And please keep in mind that participating players should also have appropriate bandwidth. Even if the server is on 10MB upload, if you have many dial-up players that may still lead to serious lag or even crashes.
Additionally, what can really eat up bandwidth is full-auto weapons fire (especially with tracers), and lots of grenades, rockets or other explosives. And last, but not least, ping response times of each participant play an important role, too.
Hope this helps.