Yeah, it's not so much "how many?", but "which ones?".
Anything streaming video is going to be using lots of data. Google Earth downloads lots of data too. All those graphics are being downloaded from the web. So those two apps alone could be using more data than using gmail 12 hours a day.
That being said ANY app that provides you with any sort of information from the outside world is using your data connection. Email, weather, news, chat, etc. Any apps that have advertisements are pulling them from the web too, so even apps that don't seem to have any "live data" updates could be using the web. MANY of the "free" apps on the Market are ad-supported.
As was mentioned, you're on an unlimited plan so it's nothing to worry about now. But being aware of it is worthwhile. Someday unlimited data may not be an option so it would be wise to have a handle on how much data you're using and why for future reference.