From Halopedia, the Halo wiki

A navarch was a Forerunner specialist that served within the ecumene's navy as crewmen, typically tasked as multi-role coordinators and operations specialists.[1] As most warships within the Forerunner fleet required only a few living beings to oversee operations, most ships were crewed by a handful of navarchs. Led by the ship's princeps, the navarchs were protected by a larger contingent of heavily armored Warrior-Servant scutarii.[2]

Sojourner-class dreadnoughts were only crewed by fiften navarchs, led by a single princeps.[2]