Comes with 1 wheel. Does not come with bearings. If your wheel broke but the bearings are still in good condition this wheel can press right onto the bearings for easy repair. This is an aftermarket part. 
Three of these pulley wheels are located in the Glowforge: one is on the underside of the laser arm, on the right-hand side, and it guides the carriage plate belt. The other two are located on the back of each side rail of the Glowforge, guiding the side tension belts.