Do I need both left and right brackets to fix a sagging bumper?

This is the right-side bracket. For full front bumper support, you'll want a matching left-side bracket. One bracket alone will improve that corner, but pair them for balanced coverage across the full bumper width.

Will I need fasteners to install this?

The bracket uses factory mounting points, so you'll need hardware to secure it — fasteners are not included with this bracket. Check your vehicle's service manual or test-fit the bracket to confirm what size and length bolts work best with your existing holes.

Can I install this before or after the bumper cover goes on?

Install the stiffener bracket first, then mount your bumper cover over it. This gives you access to the mounting points and ensures the bracket seats properly against the frame before the cover locks it in place.