Do these lock nuts require a special key to remove?

Yes. The four lock nuts come with a key that allows you to remove and install them. Keep that key somewhere safe — losing it makes wheel removal difficult until you cut the locks off.

How do I know these will fit my wheel studs?

M12 x 1.25 is the metric standard for most vehicles, but thread pitch varies. Verify your stud size and pitch before ordering — bring a lug nut to a parts counter if you're unsure, or check your service manual.

Can I use just the lock nuts, or do I need to use all 16 standard nuts?

You use all 20 total — 16 standard plus the 4 locks. The locks are mixed in with the regular nuts, typically one lock per wheel on a four-wheel setup, with the remaining spares as backups.