Get rid of all of them, they'll only get smashed off again
But they are very easy to remove and even easier to install. If it is a rear you may want to remove the wheel to make it much easier (fronts you can just spin the wheels to access flare clamps). Also might want to give the wheel well a good clean to make access to the clamps easier. All you'll need is a 10 and 12mm socket wrench and a Philips screwdriver. Mine had a couple of rivets instead of bolts in the front flares so you may need a rivet gun and a couple of rivets, or just drill in some self tapping screws instead.
I snapped the remaining good flares on mine in half just trying to take them off due to them being clipped to the side of the truck as well as around the wheel perimeter. Watch this if removing flares and want to keep them!