How many letter tiles come in a standard English Scrabble set, including blanks?

Answer

100 tiles

There are 98 lettered tiles plus 2 blanks, and only one each of J, K, Q, X and Z. The letter distribution was worked out in the 1930s by inventor Alfred Butts, who tallied letter frequencies from the pages of the New York Times.

Source: Hasbro (verified via Wikipedia's Scrabble letter distributions)