I am a web programmer and not a game programmer/net admin so i very well may be off the mark but that's a lot. since i don't know the bandwidth consumption of the game i can't say how many users but i can take a guess. the amount of data that passes from the game to the server is not as large as you would think. i suspect what is passed is your coordinates in 3 dimensional space (x,y,z), vector information (speed/direction), and object tags for you and your phasers/whatnot. the object tags are probably a number where for your object say 0=sit, 1=crouch, etc ... while for a weapon 1=pistol,2=rifle, etc. this information is sent to the server to be passed out to other gamers. they download this information and mesh it up in their game.
since we know dial-up users can play online games the number has to be small. lets say 10k/s upload for your information and 20k/s for download. please note i am pulling these numbers out of thin air, but as i said we know dial-up users play online games without much problems except in highly populated areas. this is probably due to all the information needed to download more than their positioning itself.
presuming 30k/s a 12 g/s would allow for 400,000 concurrent users peak. since network admins never want to get near that level as it starts causing problems i suspect that they are aiming to run at 50% capacity or less with peak hitting close to 75% or less.
however i just want to iterate that these are my guesses and based upon assumptions i have made and may be entirely wrong.