Is there a limit on how many destinations or legs of a trip a paratransit rider may request in a given day?

No, paratransit providers are required to provide all eligible requested trips, not just trips to and from a single location. A person may take as many individual legs of a trip in a day as their schedule allows (factoring in pickup windows and time to complete their business), just as a fixed route user may take as many trips as he or she desires. For example, just as a person may take a fixed route from their home to work, then from work to an appointment, and then home again, so should a paratransit rider be able to. Each leg may be separately scheduled (that is, paratransit providers are not required to wait while a passenger runs into the post office, just as a fixed route user would be required to wait for the next bus).