Will this guard fit both sides or do I need to order two?

This is a single-side unit — you'll want to grab one for each corner if you're protecting both tail lights.

Do I need any extra fasteners to mount this?

Uses your factory mounting points, so check if your existing hardware is in good shape before install. If bolts are stripped or missing, you'll need replacements from your local hardware store.

Can I paint or modify the black finish?

The black finish is ready to use as-is. If you want a different color, light sanding and standard automotive paint will adhere without issue.