How many license plate frames are sold per year to the American public
in the last two years?

Unless you could contact all of the (very, very many) frame retailers
and get them to give up their sales data, I'm not sure it's possible
to get anything close to a realistic figure..

Take a sample.   Pollsters often use 300, 500 or 1500.   Count the
number of cars with frames.   Find the data showing number of vehicles
on the road.  Compensate for variables such as most dealers sell the
cars with advertising frames.   At least you'll have a rough estimate.