I probably have between 3 and 20 open typically. Once it’s starts getting beyond that I get pretty anxious to finish whatever has be opening so many tabs, so I can clean things up and reset before moving on to the next thing.
If I'm researching something, I might end up with 12ish. At that point, I start taking notes with a link so I can clear out tabs.
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.