Have you ever wondered why Linux has 6 different virtual text-terminals but only one graphic-terminal? In fact, Linux has also 6 graphic terminals, it just uses only one by default.
When you start your X-Server with the command “startx”, the system assumes, you want to send the graphical output to the first graphic-terminal, which you can access by pressing Ctrl-Alt-F7.
"startx -- :0"
is normally the same as just “startx”, it puts the graphics to the first graphic-terminal
"startx -- :1"
starts another X-Server with its output to the second graphic-terminal
"startx -- :2"
starts another X-Server with its output to the third graphic-terminal
… so we can run upto 6 different X-Servers simultaneously!
You probably already knew, that you can switch between the text-terminals with the combination Alt- and the F1 to F6 keys. Parallel you can switch between the graphic-terminals with Ctrl-Alt- and F7 to F12.