WetGeek try setting it manually in dconf editor or in terminal and check

gsettings set org.gnome.desktop.wm.preferences num-workspaces 15

maximum is 36
in dconf editor I can see 12 configurable keybindings So you can set those 12 keybindings.
path :
org.gnome.desktop.wm.keybindings

      viyoriya gsettings set org.gnome.desktop.wm.preferences num-workspaces 15

      I tried this in the terminal, changing the 15 to 12, since I don't need as many as 15. The command produced no error, but neither did it seem to have any effect. The "add button" doesn't show up for the panel.

      I even rebooted Solus Budgie on the chance that a reboot might be needed for the change to take effect, but there is still no way to add a workspace to that switcher.

      As this is apparently a known issue, which presumably will be fixed someday, I'll continue to be patient.

          algent I understand now. Still hoping for a real fix sometime.

            viyoriya make a short cut keybinding to WS 9,10,11 and 12 .

            I tried that. No matter what key combination I try to set for WS 9, it produces an error telling me it's not valid for that setting. It shouldn't be this hard.

              I don't anticipate removing the restriction for more than 8 workspaces until I can guarantee reasonable performance across all devices that could be running Budgie. That isn't likely to happen until Budgie 11. There is no work around, I enforce this restriction in the window manager itself, then that gets bubbled up to places like Budgie Desktop Settings and the workspace applet.

              Modifying num-workspaces will not affect this, Budgie will intentionally modify it and drop it back down to 8 to avoid unexpected / undesired behavior.

                JoshStrobl That isn't likely to happen until Budgie 11.

                Thanks for making that clear. My workaround is simply to use just 8 workspaces. That will work for me until 11 is released.