Ctrl+Alt+F7 doesn't switch back to X session in Kubuntu 18.04 Bionic

Other as in Ubuntu 17.10 and later, in Kubuntu 18.04 the default display manager is SDDM, where:

• tty1 holds the GUI session
  get there with Ctrl+Alt+F1 or (sudo) chvt 1
• tty2 to tty6 hold non-graphical TTY sessions
  get there with Ctrl+Alt+F2 to F6 or (sudo) chvt 2 to chvt 6
• tty7 and above are unused

Further reading:

• How can I reduce the number of TTYs? (also answers how to increase their number)
• SDDM bug report from 2014 about this, which also contains a workaround to make SDDM start on tty7

On a Linux text-mode virtual console / terminal, Alt+left-arrow and Alt+right-arrow cycle through virtual consoles.

(X servers disable that keybind, so switch to a text-mode VC first, with ctrl+alt+f2, or any VC number that doesn't have an X session running on it.

If you're ever not sure which VC something is on, or which VTs exist, use one of those to cycle through and see what you find. If there's an X session running on a VC, you will find it.