A great circle, also known as an orthodrome, of a sphere is the intersection of the sphere and a plane that passes through the centre point of the sphere.

A great circle is the largest circle that can be drawn on any given sphere. Any diameter of any great circle coincides with a diameter of the sphere, and therefore all great circles have the same centre and circumference as each other.
This special case of a circle of a sphere is in opposition to a small circle, that is, the intersection of the sphere and a plane that does not pass through the centre. Every circle in Euclidean 3-space is a great circle of exactly one sphere.

Small circle: any circle on the surface of the earth which is not a great circle is by definition a small circle, there’s unlimited number of small circles on the earth surface.

To imagine how many small circles you can make get a ball and dip it partially into the water and see how many circles you can make past between two points.