Does a oven range fan have to be on its own...

According to the manufacture instructions, you always need to run any appliance on it's own circuit. Any licensed electrician will tell you to run another circuit. We have to cover our butts you know.
That being said, I have my range hood running on a shared circuit with my porch and attic lights and one outside receptacle.
It mostly depends on what else you are running on that circuit. If you already have several things on it that you would have running at the same time as the hood I would recommend running another circuit.