Horns come in sets of 2, reflectives come in sets of 4 (1 for each side of each horn)
So if you want a full set for a single helmet the quantity you'll want is "1"
Base has pre installed 3m adhesive, be sure to wipe clean AND DRY your helmet before attaching them. Then (ideally) let them sit for a few hours (or overnight) before riding so they have time to adhere properly
