I typically have 2-5 windows open, mainly because I find that some of them effect "standby"(screen off, nothing happening) battery life.
If I'm doing a bunch of stuff I can easily have many more(usually browser windows), but I tend to close most at the end of my active session.

If I had A, more ram and B, better "active-suspend" support so I don't lose battery life, I'd have a lot more open at a time.

Of course, having a dual-core processor would help too; with 4+ browser windows -loading- at a time it gets kind of laggy, even OC'd to 950mhz.

If there is -one- thing I like about the N900's multitasking its the ctrl+backspace to show all windows. That one feature makes multitasking -so- much easier - though it does require the keyboard to be open.
For a new device, I'd like to see at least one multi-function button on the top or side that would let me do that without opening the keyboard, which would make it easier to do when holding it in portrait mode.

-Rob