Or HaChaim on Numbers 17:13/אור החיים על במדבר י״ז:י״ג

1ויעמד בין המתים ובין החיים. He stood between the dead and between the living. This expression can best be understood with the help of Tanchuma on Tetzaveh paragraph 15. The Midrash explains there that Aaron took hold of the angel of death physically and prevented him from killing. At the same time he counted the number of dead to make sure that the angel of death would not add to their number. The correct translation of our verse then is: "he stood, i.e. arrested or limited the number of dead that had died from amongst the living." This explains why the Torah added that at that moment the number of dead from the plague was (already) 14,700. No additional people died once Aaron offered the incense.