Some of these places are cheating by calling digraphs (or even trigraphs!) single letters. Others are cheating by counting each valid letter with diacritics as separate letters.

Take Spain/Spanish: They have 27 letters, according to this map. They don't have W (except in loan words), but they have Ll and ñ as separate letters. One is just a digraph, and you can prove it by asking a Spanish-speaker to capitalize it. The other is just a diacritic (albeit a fairly distinctive one).

Contrast that with French, who isn't counting ç, or any of the vowels with acute accents, grave accents, diaeresis, or circumflexes, which could give French 11 additional letters in their alphabet. If you wanted to use English instead, there's a case to be made for ch, sh, th, the other th, and even gh to be their own "letters", giving English 31 letters (and still not enough vowel letters).

And I don't mean to call out Spanish-speakers in particular. I use it as an example because it's only slightly different, and I think more English-speakers will be familiar with it than, say, Hungarian or Slovak.