I can't imagine finding use with more than three. But when I have three monitors, I do this:
Main monitor: main window, maximized. Maximum work area, no sidebars (except a few pinned ones that aren't used much, and stay out of the way).
Secondary monitor: all the panels needed for editing and navigation, tiled in narrow vertical strips. Properties is closest to the main window. Next is a stack of Filter, List, Messages and Compiler. Next is Project. If I'm using other panels, like PCB, they go here as well. Next, if used: Storage Manager.
Dialogs are generally positioned over the main window (unfortunately they have a tendency to wander, anyway). Some dialogs are less important since AD18 (e.g. object edit), but Project ECO, Parameter Manager, Polygon Manager, etc. are big and important. (Parameter Manager usually gets stretched to full screen width, sometimes across several screens if I need to.)
Tertiary monitor: anything else. If I need dual window editing, I can drag off a file and reference it there. Other dialogs and panels. Supporting tools -- browser (shopping for and looking up datasheets of components, reading the forum---oh wait, etc.), calculator, uh terminal maybe I'm testing a project, etc.
Layer Panel (AD18+) also goes here, if there's no room left on the 2nd monitor.
Note that each configuration must be set for each open file type. Since AD18+, whole panel configurations change, so it's a PITA to get their positions consistent between modes.

Repeat for each of SCH, PCB, SchLib and PcbLib.
One difference: Sch and SchLib List is not enabled. It runs fucking slow. Also set PCB and PcbLib List to "selected objects". Considerable time is spent populating the full list with every click. (There are other stupid speedup tricks but these are relevant to present discussion.)
For single monitor use, I tend to keep one sidebar and flip between the panels as needed. It's a bit slow, so the 2nd monitor adds a lot of value. The 3rd monitor doesn't add much value, but doesn't need to be much added cost -- the case I have is two HD monitors (DVI and VGA) and the laptop's main monitor (docked). Laptop screen of course is smaller, which makes it harder to work between, so it tends to serve as the tertiary monitor.
Tim