Question

how many principal plane does a system of two lenses have​

Answer 1

Answer:

two principal planes

That approach allows you to calculate the locations of the two principal planes H1 and H2 and the relevant focal lengths which are associated with the lens system. The locations and separation of the two principal planes depend upon the details of the lenses and their relative locations.

Answer 2

two principal planes

The two principal planes have the property that a ray emerging from the lens appears to have crossed the rear principal plane at the same distance from the axis that the ray appeared to cross the front principal plane, as viewed from the front of the lens.

Hope it helps...