There are 264 13-letter words that start with 'un'.
Note: These 264 13-letter words that start with 'un' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with und, unc, uns, uni, una, unb, unr, unp, unf, unt, unm, une, unw, unh, unl, ung, unk, unn, uno, unv, unq, unj, unu, uny, unz.
3-letter words start with 'un' (1)