According to dictionary.com, a letter is "a symbol or character that is conventionally used in writing and printing to represent a speech sound and that is part of an alphabet." Again, according to dictionary.com, a number is "a word or symbol, or a combination of words or symbols, using in counting or in noting a total." Given these definitions, the symbol "2" is a number and the symbol "B" is a letter.
Letters and numbers should not be a difficult concept. I learned my letters and my numbers before I started kindergarten. Most people learned their letters around that same time in their lives, and I am quite sure that schools, while severely degrading in quality as time goes on, still teach letters and numbers.
All that being said, IT DRIVES ME CRAZY when people insist on naming NUMBERS on their eye exams when I repeatedly ask them to read the LETTERS on the screen. Letters, people! "2" is not a letter! "8" is not a letter! "1" is not a letter! I get that you can't quite see the symbols on the screen because you're old or blind, or whatever, but when I ask you to read the LETTERS, at least use letters in your guesses!