I am looking for some data on the (average) number of seats per car. I know one could assume such number to be 5, but I would like to have some statistics to support such an assumption.
I am not sure if you will find a statistics for this, but I assume it will be easy to do it by yourself. There are a few datasets out there that contains this info.
The Comprehensive Cars (CompCars) dataset link
Each car model is labeled with five attributes, including maximum speed, displacement, number of doors, number of seats, and type of car.
World Data Atlas link
This one includes several datasets related with cars and production. You could try some of them to see if they have the information you want.