The sons of Jeduthun - six - That is, six with their father, otherwise, there are but five. Hence it is said, they were under the hands of their father Jeduthun, who prophesied with a harp, etc.
SDA Bible Commentary (limited)
Volume 1-7
3. Six. Only 5 names are here listed. The list in vs.9-31 suggests that the missing name is Shimei of v.17 (see on v.9). The LXX includes the name Shimei, placing it 4th in the list.