Questions like "How many tennis balls can fit in a plane?" are a waste of time because they are unanswerable. To have any chance of giving a correct answer, too many variables would have to be specified to make it possible.

Ferinstance:

"How big is the plane?" "What kind of plane?" "Will the plane be flying or not?" "Will the plane require space for a pilot?" "Will the plane, if flying, restrict the number of balls such that the plane is not overloaded past its maximum All Up Weight?" "How heavy is each ball?" "How large is each ball?" "Do we fill all cavities within the plane, such as wing tanks, wing cavities, underfloor cable cavities, etc?" .... and so on.

Smart-arse answers, while showing some lateral thinking, are only comparable subjectively by the examiner. Would a different examiner judge another answer better than this examiner does? In that case would person B get the job from examiner B instead of person A getting the job from examiner A?

As I said above, these questions are a waste of everybody's time.

Anyone interviewer who asks this question should be fired.

This shows a lack of respect for the person being interviews. It shows a waste of company time and effort. It shows a lack of critical thinking by the interviewer.

Planes are infinite. Assuming the simplifying assumption that tennis balls are 2D, there are an infinite number of them.