One, unless you suppose they're descending through planes, then infinite. But one that they both occupy.
none
There is no "the" since there are infinitely many non-parallel planes. Two non-parallel planes are planes that intersect in a line.
A triangle can have no parallel lines.
A pentagon has no parallel lines.
it has too parallel lines!
A cone does not have any parallel lines
how many sets of parallel lines are in a triangle
A square has 2 pairs of opposite parallel lines.
A cube has exactly 6 parallel lines.
2 pairs of parallel lines
A cube has exactly 6 parallel lines.
There are 0 parallel lines that a triangle has.