Complex sentence: Jamborees are held once every four years, and are abbreviated as "NJ", or including the number of the event, as (for example) 14NJ for the 14th Nippon Jamboree.
Split into simpler sentences: Jamborees are held once every four years, and have currently been held 14 times. They are abbreviated as "NJ", or including the number of the event, as (for example) 14NJ for the 14th Nippon Jamboree.