How many states have counties named for thomas jefferson?

I believe that the answer is 18, I'm not too sure but 18 is my best educated guess.

Sequoia national park  /  inyo national forest, california, u.s. mount whitney is the tallest mountain in california, as well as the highest summit in the contiguous united states and thesierra nevada—with an elevation of 14,505 feet (4,421 m).