How many 12-letter words are there?

11,358 in the Tournament dictionary, which is the public-domain ENABLE word list. The Broad dictionary adds more.

What is the highest-scoring 12-letter word?

RAZZMATAZZES at 50 points, counting tile values with no board multipliers.

No. The page shows the 300 highest-scoring; the pages linked below break the full list down by starting and ending letters.

11,358 words are exactly 12 letters long. RAZZMATAZZES scores highest at 50 points.

Showing the highest-scoring of 11,358 words. The pages linked below split the rest into smaller lists.

12-letter words number 11,358 in all. Below is the high-scoring end of that list, with links to narrower pages underneath.

From 12 letters to 12: the 12-letter band is the densest with 11,358 words, and the 12-letter band the scarcest with 11,358.

The highest-scoring answer is RAZZMATAZZES at 50 points in Scrabble. It tops both tables: 51 in Words With Friends as well.

At 12 letters, RAZZMATAZZES, QUIZZICALITY and HYDROXYZINES are as long as this set goes.

To go further, put a few of these letters into the solver above and filter by length.

embezzlement: the fraudulent appropriation of funds or property entrusted to your care but actually owned by someone else
katzenjammer: loud confused noise from many sources
homozygosity: the state of being homozygous
hydrolyzable: capable of undergoing hydrolysis
quixotically: in a quixotic manner

How many 12-letter words are there?: 11,358 in the Tournament dictionary, which is the public-domain ENABLE word list. The Broad dictionary adds more.
What is the highest-scoring 12-letter word?: RAZZMATAZZES at 50 points, counting tile values with no board multipliers.
Are all of them listed here?: No. The page shows the 300 highest-scoring; the pages linked below break the full list down by starting and ending letters.