The correct answer to this open question is the following.
The territory that had missions established by Junípero Serra that housed more than 18,000 Native American converts was California.
Junípero Serra (1713-1874) was a Spanish priest that belonged to the Franciscan Mission that worked in the territory of California to evangelize the Native American Indians that lived there. Indeed, after he evangelized in México, he established the first mission in California in 1769. The place, San Fernando de Velicatá. Then, he founded another mission in San Diego. For his actively live as an evangelizor, Junipero was canonized by Pope Francis in 2015.
The highest concentration of cities probably emerged along the two rivers the Tigris and the Euphrates rivers. This would have happened because of the large amount of fertile soil by those two rivers.
