There are 206 13-letter words that start with 'su'.
Note: These 206 13-letter words that start with 'su' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with sub, sup, sur, sul, sun, suc, sum, sus, suf, sug, sud, sui, sua, sut, sue, suk, suz, suq.
3-letter words start with 'su' (7)