Just like humans...70,000 years ago after the Toba Explosion our population was at 1,000 people, the lowest it has ever been. So how did we come back from that? It's simple: incest
1,000 people (males, females) mating is far from 1 male to all the female
Those people will be far far away relatives (which isnt considered incest in most places) when those baby tortoises are all half brothers and sisters
Those people will be far far away relatives (which isnt considered incest in most places) when those baby tortoises are all half brothers and sisters