Is David the eighth son or the seventh son? 1samuel 17 v12-14 indicates the youngest of eight. 1 chronicles 2v13-15 indicates the seventh of Jesse.
3Discussed by ibn ezra here. parsha.blogspot.com/2015/06/…– josh waxmanJun 30, 2015 at 11:51
Rashi in Divrei Hayamim 2:15 says that David was the seventh born and the count stopped at David and did not include his younger brother Elijah because David was the one anointed as king.
In Samuel the term used is קטן (smallest) not youngest even though the translation is often "youngest".
Rashi in Shmuel says that it means that he was the "small" one not the "youngest" and it refers to his humbleness and position in the family.
Note in 1 Samuel 16:10
And Yishai presented seven of his sons before Shmuel ...
Again we see that David was the eighth son but was regarded as the least of the family, not necessarily the youngest.
See the comment of Rabbi David Kimchi (Radak) citing Ibn Ezra on the verse in Chronicles which you noted. He explains that Eliyahu, the 'eighth son' was sired from a different wife. David was the seventh son of the first wife. (Also see the comment of the Malbim, ad loc.)
Rashi to Shemuel Alef 17:12 suggest that one of them died without children and therefore he is not mentioned in the genealogical list of Divrei HaYamim.