While the amount of stickers you use is entirely up to you, you luckily have a tremendous amount of options on where to actually place them. The most popular part of the car where vinyl stickers are most commonly placed is between the taillights since this allows them to be noticeable without standing out too much.  This works the same when you attach them to the windows or windshields, however if you do want them to be a little more noticeable, it’s never a bad idea to place vinyl stickers on the doors for everyone to see, especially if the stickers are a little bigger.