I have voted 6-9 but is important to notice that it's because some of the applications have more than one instance (Web browser, Conversations, XTerm,...).

If more applications support tabs I won't usually open more than 4 or 5 applications at the same time.

I also make use of Desktop Switching to change between Mobile Computer / Phone / Driving desktop layouts which is very useful and avoids launching certain applications as you can use just widgets.