I usually have around 400 tabs open across 10 windows. They build up because I have many things that I intend to read, use or reference but I don't want to stash them and forget to read them like with Pocket or normal bookmarks. So I keep them open in a window so that I can return to whatever I was researching or working on some time soon.

Just an example: I'm working on an ML model, I begin by researching prior work. I get down to 100 or 200 related papers in google scholar. I start going through the papers until I find the ones that are closest to what I'm looking for. I leave the tabs of the 10 or so papers that I have found open. Now I have 12 tabs open: 10 related papers, my specific google scholar search and my general google scholar search. As I go through the related papers I will happen upon different ideas, libraries/tools or referenced papers that I want to return to. These tend to build up and you can easily get to 6 or 7 per paper. By the time I finish 4 related papers I could have 40 tabs open. This whole process happened over the course of 30-40 minutes. I also tend to find new keywords to use in google while I'm reading these papers which I then produce more google scholar searches from and repeat this same process over again. Now imagine having done this many times over with various projects you're working on but not necessarily required to finish. That's how you end up with a few thousand tabs in onetab and 400 tabs open in chrome.