I usually open 1-6 apps while I'm using the N900, depending on my needs.
Example, while I'm in class, I usually run
- Elements touch
- MathJinni
- Notes
- PDF Reader
- Web browser/Opera Mobile
or while I'm on traveling, the apps will be
- GPS Recorder
- one of
> Ovi Maps(Just for locate myself because the feature allow only this...)
> Sygic(For nav, but I don't much like the interface...)
- Media Player or FM Radio
- Conversations for IMs & SMS
- Email app
- A browser

PS: running more than 6 apps doesn't affect the N900, but affect me!
maybe my brain's "RAM" is less than the N900's.