Sold as a set of four pairs which is enough for two boots. Flat rivets can be used to attach the back strap to the inside of the boot permanently. They are especially useful if your horse has a narrow gait and tends to brush feet to each other, which can cause the back straps to come undone when the standard ball rivets are used. Flat rivets also leave only one end of the strap to attach when booting so you will have a left and right boot. Many riders enjoy that option.
By attaching the back strap with the flat rivets and cutting off any extra strap beyond the rivet closure will ensure the strap stays closed. But keep in mind that opening the flat rivets requires a screwdriver so is not as easy as when ball rivets are used. Therefore it's important that you get the fit right when putting the flat rivets on, that way you do not need to open them during normal use of the boots. For additional staying power, I recomment using Locktite thread locker blue 242. I put a dab in the cap end of the rivet and also on the thread for extra security. Have paper towels handy. Don't ask me how I know.
Here is a video showing you how to attach the flat rivets.
Flat rivets are used as an alternative to ball rivets, to attach the back strap to the inside of the boot permanently.