The belts used in the safety demos are also the extenders. There are multiple ones on each aircraft. (Even planes that have video demos come with back-up demo belts, oxygen masks and life vests in case the video demo doesn't work.) So they definitely do not need to be reserved in advance.

Many people don't even realize they need one until they get on a particular aircraft because the regular belts vary in length so much - people may easily be able to buckle up without an extender on one aircraft, change planes and then realize they need one on their connecting flight.