One.
There's actually two...
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There can be only one pair of perpendicular lines in an isosceles triangle when it is a right angled isosceles triangle. A non right angled isosceles triangle has no pairs of perpendicular lines.
a triangle, pentagon and that's all i can think of * * * * * A right angled triangle has a pair of perpendicular lines! Likewise, a pentagon can have a pair. A circle, ellipse etc do not.
two if the triangle has a right angle
Right angle triangle, yup that's right
Two