Depends on what they are all doing. just surfing the web quite a few. Downloading large files, not so many. Playing games on line , depends on the game.
512Kilobits/s can theretically give transfter speeds of about 64Kilobytes/s. That might give you a better idea of how many users you would want to put on it
I really couldn't tell ya exactly how many users would work well on that. I ohnestly haven't ever used any connection below 1.5-3Megabit at home or below 45Megabit at a work place. I guess I'm just spoiled, lol.
There are software apps and network apliances avaliable that would be able to allocate that evenly. Such as restricting internet speed to 4-5kilobytes/s by user and/or ip. Lots of free linux router distros have that option and it works decently.
If it is ONLY email and web surfing (no downloading) you could probably put 20 people on that comfortably. That would be for constant use. If use is very off and on with few people on it at a time you could probably get upwards of 50 users or more on it.