the looping after linking the path of the camera with the circle curve, ends at 100. How can I change that to 250 or just infinity (infinite loop) ?
Instead of using a circle curve (which is much more complicated) i would propose to add an empty at the place where you would normally have the center of your circle. Then make the empty the parent of your camera. Then just animate the rotation of your empty -> and you got a perfect circle for your cam movement.
For the animation e.g. if you want to animate the z-rotation value, go to frame 1 and hover of the z-rotation value of the empty and press I. Then go to your last frame, hover of the z-rotation value and enter 360. Press I again.
So you have an infinity loop if you play the animation.
To make the looping cycle perfect hover over the timeline, press V and then choose linear so that the rotation speed is always constant.
if you want it your way, you can change the frame in the curve here:
Path animation -> frames