Harry sets up his Teenage Mutant Ninja Turtles in a big circle, spacing each turtle at an equal distanve from?

Harry sets up his Teenage Mutant Ninja Turtles in a big circle, spacing each turtle at an equal distanve from its neighbor. He then begins counting them in order around the circle, but loses track of where he started before he finshes counting. Then he notices that the sixth turtle is directly opposite the seventeenth turtle, and he realizes that he can still figure out how many turtles are in the circle. How many turtles are in the circle.