SKS Secu-Clip, each
The practical and Handy thing is that they allow you to take your fenders off faster and more easily. Although only intended as a Safety thing for the fork, there's no rule against using them on the rear for easy fender removal.
Popping off the fender stays sans tools makes taking the rear wheel out of rear facing track style dropouts (like on the SimpleOne) tons easier. The SKS fenders and hardware kits already include clips but if you don't need to buy everything else we now offer the clips by themselves. And being plastic they do break from time to time.