Note also - use of that terminology is inconsistent; prior materials have potential for multiple.
Invasion Earth shows 3 starports.
As for MT:WBH (page 74), each major city in a mainworld may have a starport as good as the one in the UWP, while minor cities can have lesser ones.
Also according to the same book, if TL is 5+ each primary, secondary and tertiary city has at least an H rated spaceport (I guess the airports can be soused, and that is what they are refering to), and if TL is 10+ each ssecondary city has st least an F rated spaceport.
Earth, in MT S&A has 3 primary cities, each with an A rated starport, 30 secondary cities with C rated ones and about 750 tertiary cities with G class spaceport
Kusyu has 5 major cities, 3 with A rated starports, 1 with a B rated one and the other with a C rated one. Its 22 secondary cities keep having C class ones, and the 79 tertirary ones G class spaceports.
According V&V, Vland has 2 major cities (A spaceports), 22 secondary ones (Spaceports F) and 105 tertiary ones (with spaceports H); while Lair has 4 major cities (one starport A, 2 Bs and 1 C), 16 secondary cities with F spaceports and 64 tertiary ones with H rated spaceports.