RTFM The guides have a lot of good info. The calculator link no longer works, but the formula is still the same and you can do the math fairly simply. 100M would be 1000000 for bandwidth (upload speed) using the formula in the guide. Your download speed is not that important for a server.
You can adjust both the number of players and sv_maxrate. I have seen max rate set from 2000 to 10000. I think the default was 8000. You can play with it and see how it affects game play. A lower max rate supports more players but can degrade the smoothness of game play as perceived by the players.