In Linux, there is a pseudo-teletype multiplexor which handles the connections from all of the terminal window pseudo-teletypes (PTS). The multiplexor is the master, and the PTS are the slaves. The multiplexor is addressed by the kernel through the device file located at /dev/ptmx.
The tty command will print the name of the device file that your pseudo-teletype slave is using to interface to the master. And that, effectively, is the number of your terminal window
