After searching on the internet, I got some information:
- To open a tty, the file /etc/inittab is the key
- Ubuntu has no inittab, which has been replaced by the files in /etc/event.d
# tty1 - getty
# This service maintains a getty on tty1 from the point the system is
# started until it is shut down again.
start on stopped rc2
start on stopped rc3
start on stopped rc4
start on stopped rc5
stop on runlevel 0
stop on runlevel 1
stop on runlevel 6
exec /sbin/getty 38400 tty1
Ah, the rule emerged. I copied the tty6 to be tty8 to tty12 (tty7 is for the GUI desktop, as I know...), then replaces the string ``tty6'' in all the copies to their corresponding tty number. Finally, I have 11 virtual consoles, i.e., tty1 to tty6 and tty8 to tty12.