First rule of owning handcuffs: stash extra keys. Lots of extra keys. Make sure you have a long-handled key for when they’re behind your back. Second rule: Learn to pick the lock for when you don’t have a key.
I have a few pairs of handcuffs, but not for the reasons above. I carry them (or have them handy) whenever I carry a gun, which is pretty much all the time. Seems sensible to me that any situation where I draw a gun in self defense is a situation where instead of shooting somebody, I might well end up holding somebody at gunpoint. At that point, it’s important to get control of them immediately. It’s hard to call 911 with somebody at gunpoint, without dropping your guard. Then it will take what seems like hours for the cops to get there. A pair of cuffs can help solve this.
I’ve got a pair in my vehicle, and a couple pairs at home... And keys stashed all over. I also have some zip-tie cuffs. They’re almost as good and work well in a pinch.
Seems awfully tricky handcuffing someone with one hand while they are trying to take the gun out of your other hand (one ping only, hehe).