Obviously, a driver of a vehicle with a hostile or aggressively-toned sticker should set off alarms: it's adaptive to be wary of someone displaying the message:
This Car Protected by Smith and Wesson
But not necessarily:
Commit Random Acts of Kindness and Beauty.
But what about the number?
Paul Bell, Lucy Troup, and William Szlemko reported in the Journal of Applied Social Psychology that drivers who plaster their vehicles with more stickers are more prone to road rage than drivers who leave their car or truck unadorned.
It’s related to territoriality. Researchers have long known that drivers who have a strong sense of personal space while in their vehicle are more likely to be road-ragers, and the more someone plasters his vehicle with bumper stickers and decals tend to be more territorial about the areas around their cars.