Depends on what your question means. It is either an infinite number or two. The number 2 comes from the fact that there is 1 right angle on either side of the diameter. The end points serve as the end points of the right angle. The vertex of the right angle is on the circumference of the circle.
However, the vertex can be anywhere on the semicircle. Thats where you get the answer of infinite.
Without refinement in the question, I don't know which answer to choose. I suspect you should try 2 first.