>   xfce4-session            1825
>   kde-plasma-desktop        540
>   mate-desktop-environment  489
>   lxqt-session              293
>   cinnamon-session          282
>   openbox-lxde-session      171
>   gnome-session             132
>   openbox-gnome-session       3
>   openbox-kde-session         2
>   lxqt-wayland-session        1
>   xfce4-wayland-session       1

Here are all WMs, which I found using "apt-cache search 
x-window-manager":
   xfwm4                    1999
   kwin-x11                 599
   marco                    540
   openbox                  340
   muffin                   292
   cinnamon                 274
   i3-wm                    144
   metacity                 108
   fluxbox                  98
   icewm                    82
   wmaker                   67
   fvwm                     64
   jwm                      47
   awesome                  45
   twm                      39
   enlightenment            37
   dwm                      36
   mutter                   30
   bspwm                    25
   cwm                      22
   sawfish                  19
   ratpoison                17
   stumpwm                  16
   blackbox                 16
   pekwm                    12
   mwm                      10
   herbstluftwm             10
   aewm++                   10
   matchbox-window-manager  9
   flwm                     9
   evilwm                   9
   afterstep                9
   vtwm                     8
   lwm                      8
   icewm-experimental       8
   fvwm3                    8
   fvwm-crystal             8
   spectrwm                 7
   notion                   6
   miwm                     6
   wm2                      5
   ctwm                     5
   ukwm                     4
   windowlab                3
   w9wm                     3
   clfswm                   3
   subtle                   2
   fvwm1                    2

And these additional ones, which I happened to remember and weren't in 
the list. Some of those are wayland shells, instead of X11 WMs, though. 
Not a lot of wayland shells.
   lxde            143
   sway             57
   dwm              36
   weston           26
   wayfire          12
   xmonad           10
   hyprland          3
   phoc/phosh        -

Some more I found after more searching:
   bambam                14
   impressive-display    12
   mir-demos              2
   sugar-session          -
   sxmo-utils             -
   lomiri-desktop-session -

And for the display managers:
   slim     1610
   lightdm  1028
   sddm      691
   gdm3      175
   xdm        88
   lxdm       47
   wdm        29
   nodm       10
   ldm         5