I have not been able to get the current workspace number permanently on the desktop. If I press the window key, of course I can get it, but find this annoying. I have 4 fixed workspaces, set by gnome-tweaks. I note that Fedora 28 Gnome has the workspace number on the status line ( showing 2/4 to indicate workspace 2 of 4).
Install the following gnome extension:
Or install from Ubuntu package with the default Gnome Shell extensions - it's probably more reliable.
You can install it using:
sudo apt install gnome-shell-extensions
After installing that package, restart Gnome Shell using Alt+F2 and typing r, then press the Enter key.
And finally enable the "Workspace Indicator" extension using Gnome Tweaks.