1200 GB is a lot of data transfer. Still, it depends on how popular you expect your site to become, but also, what your site delivers. Are you streaming video? Do you have a lot of images?

Just as an example, say each browsing session per user consumes 10MB (which is quite a bit, in terms of web browsing). You've got 1200 x 1024 = 1,228,800 MB to use.

Axe that to a nice round 1,000,000 MB to allow for other data flow between the server, you've still got room for 1,000,000 / 10 = 100,000 user sessions.

Hosting experts, please criticise if I don't know what I'm talking about