The Greek alphabet currently has twenty-four letters, and this has been standard for millennia now. However, three extra symbols are used for numbers, and other answers mention letters like tsan used only in specific dialects.
How many letters are known to have been used in the "Greek alphabet" over the years? (And, if possible, what are they all?)
(Note: by "Greek alphabet" I mean all the varieties used outside of Italy, up until the Latin alphabet took over. So Latin and Etruscan don't count, and Cyrillic doesn't count, but pretty much any variety of Greek does.)