the definition of "alphabet" is a set of letters, arranged in a particular order, and used in writing

From the blog written by Prof Zuke,

Japanese EFL teachers refer to each letter in the alphabet as "an alphabet," which is a mistake. A is not an alphabet. B is not an alphabet. They are the first letter, the second letter in the modern English alphabet, respectively.

This is good lesson for me.
I sometimes said so.  Alphabet is a set of letters.