OK:
1. Do we include the ampersand as last letter of the English alphabet? https://en.wikipedia.org/w/index.php?title=English_alphabet&...
2. Do we include arachaic letters? https://en.wikipedia.org/w/index.php?title=English_alphabet&...
3. Do we consider diacritic letters as own letters?
3a. Do we include diacritic letters that only appear in loanwords? https://en.wikipedia.org/w/index.php?title=English_alphabet&...
3b. Do we include diacritic letters that were used in older English writings to indicate the syllables? https://en.wikipedia.org/w/index.php?title=English_alphabet&...
3c. What about diacritics that appear in orthographies of regional dialects? https://en.wikipedia.org/w/index.php?title=English_terms_wit...
For 3, see also https://en.wikipedia.org/w/index.php?title=English_terms_wit...
4. What about ligatures that were in use in English until the 19th century, but are today much more rarely used? https://en.wikipedia.org/w/index.php?title=English_alphabet&...