The current ACSM Guideline for Muscular Strength and Endurance programming is 2-4 sets for each major muscle group.
Program your circuit class based on the type of participant, age, goals, etc. They is no one golden rule for circuit.
To slow down boredom, I do different formats.
*8-10 stations, 2 sets
*20 stations, 1 set.
*8-10 stations, 1 set followed by another type of 8-10 station, 1 set