At the moment the setting from X session regarding the setup of virtual desktops is used in Wayland. But it's not changeable again in the KCM (here it only says 1 screen and 1 row all the time). The pager consequently only shows one virtual desktop. Reproducible: Always
Same here, but a few additional bits of info: Under wayland I can change the screen/row values (arrow keys) The number of desktops available is not in any way influenced by the screen/row values while under wayland. Reguardless of screen/row numbers, the desktop pager is blank and doesn't do anything. However, the key bindings I have set up to change desktops does work. So the Desktops are there, just the pager widget fails to be of any use.
Yes, the KCM is not ported at all yet and still interacts too strongly with X11.
See also : https://bugs.kde.org/show_bug.cgi?id=368675
Confirmed on Fedora 28, kwin 5.12.5. I can't change the number of virtual desktops nor any of the other options in kcm_kwindesktop.
There is currently work being done to enable this feature again: https://phabricator.kde.org/D12820 But this is only the beginning and more needs to be done. This might be ready for 5.14.
AFAICT, this has been fixed in the mean time. If this still doesn't work for you, please reopen.