If this were an actual question referring to the actual alphabet, the answer of 26 will be correct.
If this was a question directed riddle-wise, 'the alphabet' will have 11 letters.
The answer that c-a provided (7 letters) is not right.
Why?
The question asked "how many letters are..." It never stated anything about repetition of letters.
A letter is defined as: a symbol or character that is conventionally used in writing and
printing to represent a speech sound and that is part of an alphabet.
So the letter 'T' is a letter. It doesn't matter how many times it is repeated: 'TTTTT' is 5 letters.
So long as the letter is a symbol or a character, it is considered as a letter, no matter how many times it has been repeated.
I think I prove my case.