I haven't played much SP, only did the first 3 missions so far. The car variety has been great from what I've seen. When I go online it varies depending on how many players there are on the server. If I do invite only or solo, the variety seems okay but I still usually see two or three cars continually repeat in traffic until I enter a new area where a new group of vehicles will continually reappear everywhere for a while.
It's annoying but doesn't bug me that much as other issues I see in the game.
The bug seems to happen sometime after the 70% completion mark in sp. Online is a seperate matter. I've pretty much given up on playing my first save file for now (over 85% complete).