I usually have 4 windows on my 34" QHD monitor (16:10 aspect ratio), the fifth window is usually a full-screen window on a different workspace or monitor.
A stacking WM is a “normal” WM that most people use, like Mutter (Gnome) or kwin (KDE). Also called as floating WM. It’s called stacking because windows are organized in a layered stack, one on top of the other, similar to pieces of paper on a desk. They have a “z order”, and can be “above” or “below” each other, along the Z axis in the stack.