The Terminations panel in the Graph Tab lists the normalizing impedance (Z0) for the ports presently plotted in the response viewer. If the same normalizing impedance is being used for all the ports, there is only one entry defining the impedance for all the ports. If you have selected multiport and defined different normalizing impedances for different ports, then the word "Multiport" appears in the panel and the normalizing impedance of each port is displayed.
To change the normalizing impedance for a port, please see the Terminations page of the Graph Options dialog box.