Spanish speaker here. "ch" and "ll" being separate letters has been discussed for a long time and finally the decision was that they weren't separate letters but a combination of two [1]. Meanwhile, "ñ" stands as a letter of its own.

Accented vowels aren't considered different letters in Spanish because they affect the word they are in rather than the letter, as they serve to indicate which one is the "strong" syllable in a word. From a Spanish view of point, "a" and "á" are exactly the same letter.

[1] http://www.rae.es/consultas/exclusion-de-ch-y-ll-del-abeceda...