How many cars and lightposts are on the back of a US 10 dollar bill?

The current $10 bill shows two lightposts and no cars.

Before the bill was redesigned in 1990, the vignette of the Treasury Building showed 8 lightposts and 4 identifiable cars, although a 5th may be barely visible.

The Treasury is firm in its statement that the cars were composite images, in order to avoid favoring any particular manufacturer. Many people who have looked at cars of that period have found elements that appear to have been taken from a Ford Model T and a 1926 Hupmobile, among others.

The same design persisted for almost 60 years and became emblematic of the Treasury's then-refusal to update American currency. When that policy finally changed in 1990, the new image no longer included anything that could be obviously be associated with a specific time period.