There is no limit on what you can drop, the way you drop trips is via trip trade. Basically the trip you are wanting to drop has to be either placed in open time and that means it's just in the trip pool unassigned. For that to happen there has to be sufficient reserve coverage to allow a trip to drop into open time. The other way you can drop a trip is someone has to want that trip and pick it up from you, essentially like a one way trade. So how much you can drop depends on reserve coverage and/or some other pilot wanting your trip. If you are trying to drop a crummy trip on the 4th of July and your fleet/seat/base has low or no reserve coverage on that particular day you will have no chance of dropping it. If you have a commutable 3 day trip day flying that pays high on weekdays and is commutable on both ends and there is plenty of reserve coverage, well your chances are much better.