Do I need to buy the right side headlight too?

This is the left side only. If you want to replace both sides, you'll need to order the right side assembly separately. Many people do both at the same time for a symmetrical look.

Will I need new bulbs for this headlight?

Your existing bulbs should transfer over if they're still good. Check the bulb type for your vehicle in the service manual to confirm compatibility before installation.

Does this assembly mount with factory bolts?

Yes—the assembly uses factory mounting points, so your existing hardware will work. No drilling or custom brackets required.