If it's a right triangle, then one.There is no other way to have a perpendicular angle in a triangle without it being 90 degrees, or right.
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The 2 perpendicular sides of a right angle triangle intersect each other at right angles
They meet at right angles
A right angled triangle.
Perpendicular lines intersect at right angles.
A triangle with a right angle in it is the only case in which a triangle can have perpendicular lines. Triangles themselves cannot be perpendictular unless you are referring to one of the sides being perpendicular to a certain line.