How rare is a 1976 Lincoln Kennedy penny?

Not rare and not a mint product. It's a novelty item made by a private company that took ordinary 1976 cents and counterstamped them with Kennedy's picture. They're considered to be altered coins with no numismatic value.

Carloads were made for the Bicentennial and sold as "instant collectibles" but the market fell apart as soon as the Bicentennial celebrations ended, so you're unlikely to find buyers even among collectors of novelty items.