Hi there,
As Kronbits said, if you are working with a looping animation and want to reduce the resulting frame number, first hit "Loop Animation" then decrease the "Render FPS" option. By default it renders the animation at 60fps, so a 200 frames animation will output the same 200 frames, if you then reduce the render fps to 20 it will record only 1/3 of those frames, giving a result of 66 fames. You can get lower render fps by right clicking the box and manually setting a lower number.