As described in Making the Alphabet Dance (and earlier in the February 1977 Word Ways), two English words can use a maximum of 20 different letters without repeating at least one. This is accomplished by the GUNPOWDERY BLACKSMITH who is by now assured a place in logological history.
"The Gunpowdery Blacksmith,"
Word Ways: Vol. 31
, Article 15.
Available at: https://digitalcommons.butler.edu/wordways/vol31/iss3/15