There are actually quite few. I imagine that theives and assasins have the best idea how ma y there are as they interact with them.
I would guess that each zone has less than 150 permanent npcs. Maybe the entire world has less than 2500.
The difficulty is greater if you are counting nps that only exist within quests. Most of the npcs i interact with are temporary quest characters like lady clarissa laurent / stibbons / eveli etc.
I would like someone to perform a census.