6*5/2 = 15 planes.
4 planes.
Coordinate planes
8
no planes are not perpendicular to each other because for air resistance the planes are designed in a such way that air is avoided to back side and they are inclined to each other.
Yes
perpendicular
If they meet at right angles
You can only travel 90 degrees north, which is the North Pole. The poles are in a plane (all planes) perpendicular to the equator.
Altitude
one
No, perpendicular planes intercept at only one point. Parallel planes do not intersect at all.
Yes. A line can be the intersection of two planes, or any number of planes. A real-world example would be the central axis (axle) of a paddlewheel on a steamboat, which would be coplanar with all of the planes represented by the individual paddles. A single line is similarly the intersection of perpendicular planes.