What would be the minimum number to form a sustainable population? What sort of range do individual garters cover within a breeding population?
I suspect that the number required within a relatively small area would be quite high.

I expect there are odd escapees that survive for years in the wild, but not a population. Anyone deliberately releasing enough garters to form a wild population would be both seriously irresponsible and also breaking some law or another (not sure what law it would be in the UK).