A flock of a hundred sheep was of average size for a shepherd of modest means. God’s people are often identified as the Lord’s flock (Psalm 23; Isaiah 53:6; Jeremiah 13:17; Ezekiel 34; Zechariah 10:3).
leave the ninety-nine: Other shepherds could watch the ninety-nine, so those left behind were not in danger. The shepherd would do anything to find the one that was lost.