In "the English Alphabet" there are 18 letters. In the actual English alphabet, there are 26 letters. In "English Alphabet" there are 15. In "the English Alphabet" t, h, e, n, g, l, i, s, a, p, b, are individual letters that occur at least one time, so the variety of letters would be 11. This is the same without "the". Several answers here, take your pick.