It seems as if it was a cultural ideal at the time to have seven sons (see Job 1:2; Job 42:13; Ruth 4:15). Though 1 Chronicles 2:15 says that Jesse only had seven sons, 1 Samuel 16:10–11 makes it clear that there were eight with David being the last-born.1 The reason for the discrepancy is most likely that one of Jesse’s sons died before he could father any children and so he was excluded from the genealogy in 1 Chronicles 2:1–55.2
10 And Jesse made seven of his sons pass before Samuel. And Samuel said to Jesse, “The LORD has not chosen these.”