Zion was the first and, as of 1918 PD, only settlement on the planet Refuge that qualified as a town.
The Fellowship of the Elect's settlement was low in resources (very low grade technology base, including no space capable ships at all, and little else); they were not, for religious reasons much interested in improving their technology base.
Its population of less then 100,000 people, who lived in Zion and the surrounding area. (HHA4.6: TSotS)