Trucks come as a pair. 

Size information: 

5.0 - 7.62" axle - for decks 7.9" and smaller

5.2 - 8" axle - for decks 7.9" - 8.1" 

5.6 - 8.25" axle - for decks 8.1" - 8.3"

5.8 - 8.5" axle - for decks 8.4" - 8.6"

6.1 - 8.75" axle - for decks 8.62" and bigger