How many 2-letter words are there?

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

What is the highest-scoring 2-letter word?

AX at 9 points, counting tile values with no board multipliers.

96 words are exactly 2 letters long. AX scores highest at 9 points.

The dictionary holds 96 words of exactly 2 letters. A selection is below, ordered by Scrabble value.

Lengths span 2 to 2. The middle of that range is where the words are: 96 of them are 2 letters long.

If you are playing for score, AX at 9 points is the pick. Words With Friends scores it differently and prefers JO at 11.

The full 2 letters are used by AX, EX and JO.

If you need a narrower cut, the pages linked below split this set by letter and by length.

How many 2-letter words are there?: 96 in the Tournament dictionary, which is the public-domain ENABLE word list. The Broad dictionary adds more.
What is the highest-scoring 2-letter word?: AX at 9 points, counting tile values with no board multipliers.
Are all of them listed here?: Yes, all 96 are on this page.