I run 7 monitors. I've got a 2x3 grid of 24" monitors on a stand, and then a 27" widescreen standing tall beside it for code or a long terminal.
It's fantastic, though I could stand to have another row above. It's nice rarely having to switch windows - everything I need and use is right in front of me.
I tried 2 and 3 monitors and then I realized I don't like moving my neck / chair.
I use i3 and now I have 10 monitors just in front of my eyes. I can easily switch windows, move an window to the front, side by side it's fantastic.
Usually my applications have a default windows in which they open, switching to a specific application is muscle memory now. No mouse movement, no head movement. Just eyes and fingers coordination.
I usually have 8+ opened applications at the same time (editor, multiple terminals, mails, browser, chat) everything is in its right place.
Since my eyes can't look at two thing at the same time, no need to display everything. It's easier to press the shortcut to switch windows than to move my head.
Everything fits on my 3840x2400 13' laptop, it's like a Tardis, bigger on the inside.
No honestly I'd rather everything were flush, but that's the best I can do with the stand and monitors I have. The monitors aren't all the same make/model.
Pretty much the standard tools that most people have open all the time I guess. I just want to see them all all the time without having to constantly switch windows or hide the thing I'm working on.
Music, messenger, Discord, Slack, 2 or 3 instances of my IDE, source control UI, a terminal or three, documentation for the thing I'm working on, notepad++
It's fantastic, though I could stand to have another row above. It's nice rarely having to switch windows - everything I need and use is right in front of me.