כָּל־עָרֵ֥י בְנֵֽי־אַהֲרֹ֖ן הַכֹּֽהֲנִ֑ים שְׁלֹשׁ־עֶשְׂרֵ֥ה עָרִ֖ים וּמִגְרְשֵׁיהֶֽן׃ (ס)
All the cities of the children of Aaron, the priests, were thirteen cities with the open land about them.
Read midrashic interpretation of Joshua 21:19: and classical rabbinic exegesis.