You have it backwards. There should be more of them. Exact dates aren't given how long Ariadna was separated but it would be 52 years later AFTER YJ, PanO, Haqq, and the Nomads established themselves to become the Human Sphere when Dawn was recontacted. No number is given for how people were on Ariadna (the name of the ship). Many here have given good estimates on how people you'd need to start up a colony. This wasn't just people they sent as well. They had the genetic material (read: eggs and sperm) as well for genetic diversity and probably the ability to create vat babies.