I'm not a network admin, so I have less experience in these larger decisions, but here I go anyway.
The more people using the internet, the more averages come into play. I could see a 2mbit connection working for 100 people, but sometimes the net may seem slow because it just so happened many people requested data at the same time.
But you said 2mbit per 100, so 6 times that or 12mbit. Since this is a larger number, the chance of many people requesting at the same time drops dramatically.
Personally, I would say this should work assuming there aren't too many people doing heavy browsing or hitting sites with lots of media.
If you're hosting any services, you may have to consider server traffic also.
You may also want to add some form of QOS like packet shaping to make sure no one user can saturate the bandwidth and/or reserve or dedicate bandwidth for a service.
Most of the logic I used is a mix of thinking how the network works and personal experience of watching my bandwidth while browsing. I have a 16Mbit connection at home. yes, the higher bandwidth does help make pages load faster and be more responsive, but my overall bandwidth used is actually VERY low. Most users may saturate that 12mbit connection for 2-4 seconds once every few minutes. Assuming there isn't too much overlap in user data requests, it should be fine.
Another thing I thought of is you may want a "patch" server on your network for whichever OS you use. If you use Windows, it is called WSUS. If you have 600 computers all requesting a 100MB Windows update, your network could be hosed for a good long while. The WSUS server means that server becomes "Windows Update" and this also allows you to schedule when and if a patch is pushed out. Your Windows patches would be downloaded to the WSUS server, which means they're only downloaded once, then all your client machines will download from this internal server, which reduces external load.
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