What factor limits the number of eclipses per year?

The tilt of the moon's orbit around the earth

Explanation:

On average of twice a year, throughout it's orbit, the moon will create a 180° angle with the Earth making the Earth's shadow visible on the surface of the moon causing a solar eclipse.

explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.