I usually have between 1 and 3 apps. I'd use more occasionally, but the N900 is a bit memory-starved and gets painfully slow if I open too many of them, so I got into the habit of closing one of the three running apps before opening a new one.

I guess the apps I use are more memory hungry than average (FBReader, xterm with vim editing a large'ish text file, and especially the web browser).