The Arabic alphabet is usually given as twenty-eight letters, and the count is a convention rather than a fact about the marks on the page. Several things that are written and read are not among the twenty-eight.
Alif maqsura is a final ya without its dots, standing for a long a. Ta marbuta is a final ha with the two dots of ta, closing most feminine nouns and pronounced as t or as a light h according to context. The hamza, which writes the glottal stop, is a sign rather than a letter, and is carried on an alif, a waw or an undotted ya, or written on the line alone. The lam alif ligature is a single mark standing for two letters. Alif madda is an alif with a wavy stroke above it, marking a glottal stop followed by a long a. None of these has a numeral value in the abjad reckoning, and none is counted in the twenty-eight.
Whether they are letters is not a question with a single answer, and different teaching traditions answer it differently. Charts printed for beginners commonly show twenty-nine items by adding lam alif at the end, and some add hamza as well. The abjad sequence, being a list of values, counts only the twenty-eight.
That the count is conventional matters for anything that depends on it. A numerological claim that rests on there being exactly twenty-eight letters, or on their total being some particular sum, has taken a teaching convention as though it were a datum. The letters that carry values are the twenty-eight, the values are those of the inherited sequence in one of two regional reckonings, and everything else on the page is a spelling device.