There are many more Humans and ex-pets than Elvwists because a flu virus that hit the Humans during the period immediately after the Change, mutated and infected the Elvwist too. The ensuing epidemic took a number of lives, both Human and Elvwist, before being eradicated.
The virus had an unfortunate added effect on the Elvwist. They had never been as fertile as Humans, and over the centuries this side-effect has caused them to became even less so. Their villages have become enclaves within the greater society, containing the universities where most of the Humans go for higher education.