Pingry had 12 National Merit Semifinalists, Newark Academy had 7 (see the 2nd link), and Delbarton had 1 for Class of 2018.
Interestingly, Oratory Prep had 3 and is a smaller school. Seton Hall Prep had 2 and Christian Brothers Academy had 3.
It is much harder to make National Merit Semifinalist (223 index score) than National Merit Commended (213 index score).