Usually one. If I'm actively resarching something, it can go up to a dozen or so temporarily. In that situation, I'll also usually have multiple instances of the browser going to segregate topics. I tend to prefer multiple windows over multiple tabs because it makes it easier for me to navigate and keep more than one thing visible at a time.
I've been considering the multi-window approach, to leverage the system's taskbar, which is natural to use, but I don't know if I'd open more than 2-3 windows, since it would fill the space quickly (unless I use a taskbar that groups items).
Usually one. If I'm actively resarching something, it can go up to a dozen or so temporarily. In that situation, I'll also usually have multiple instances of the browser going to segregate topics. I tend to prefer multiple windows over multiple tabs because it makes it easier for me to navigate and keep more than one thing visible at a time.