Hi
from experience its about 25kbps per person but got other factors to take in to account which work out to be 600GB per month per 10 players if it full with 10 players 24 /7 which is very unlikely i have never been able to get a code 4 server to stay alive for that long
so for 20 players over 10 server for a month would be would be 35TB for the month but in reality server will be luck to be alive for 12 hours per day so it only 17.5TB per month and for the 20 player over 10 server would need a min of 4.5Mbps so 10 mbps should do the trick
with a 10mbps link would give you 25TB of bandwidth per month weather or not you hosting company will allow you to do that