Quote Originally Posted by Wraith View Post
A few of the early Horus Heresy stories mentioned a Navigator being killed and replaced in places far, far away for Terra. So presumably, Explorator fleets carried a few spares - as in, one or two per fleet, which could be dozens of ships.

Octavia was similarly a 'spare' Navigator for the Covenant of Blood in the Night Lords trilogy. That one was kind of wonky, due to why Talos recruited her and the fact that he's heretic Astartes, but otherwise no one remarks on it being weird that they have a spare Navigator.
Adding on to this, you would also get situations where a ship is wrecked in some way. One of the top priorities would be to get the Navigator off the ship. So a ship might end up with two Navigators by dint of having salvaged one, and hadn't yet made it back to a port where the Navigator could transfer off the ship.