The intersection of two planes forms a line.
A minimum of 2, but an infinite number of planes can intersect at the same line.
2
If I understand the grammar correctly, the answer is one line.
Two planes intersect at a line. The line where they intersect pertains to both planes. In the same manner, if infinitely many planes intersect each other at the same line, then that line pertains to the infinitely many planes.
A line
If 2 points determine a line, then a line contains infinitely many planes.
Given a line, there are an infinite number of different planes that it lies in.
An infinite number of planes. Picture a line and now picture planes going in every direction through the line,
exactly 1
Infinitely many planes.
Infinite.