You appear to be saying "how many possibilities" when you say "levels." When it comes to flying, the correct answer is "all of them."
Clearly it's conceivable for something to happen for which there is no procedure...one can imagine all kinds of exotic possibilities. We have no procedure, for example, for an aileron falling off the airplane. We do have procedures for jammed ailerons, jammed elevators, jammed rudders, etc...or landing with some, but not all ailerons, etc.
We have procedures available, and plan accordingly, for failures or malfunctions of any given aircraft system. All flights are planned with the worse-case "what if" in mind.
I'll be leaving in a few hours for a flight from LAX to Honolulu. You can bet I wouldn't be comfortable doing it unless I knew in advance that all the possibilities were covered for dealing with engine failures, pressurization failures, communication failures, etc...we've planned for these failures before the flight ever gets dispatched, starting with the takeoff roll and ending up with parking at the gate...and we've planned for them given real-world conditions, such as weather, winds, etc.
Rest assured that when you're riding somewhere as a passenger, the crew lives in a constant world of "what if" as we address each contingency. What if the engine fails here? What if we have a report of smoke in the cabin? What if we lose communications? What if we lose all generators? What if we encounter volcanic ash? What if...? We have a procedure and a plan for each of those...and hundreds of pages of many others, too.
Remember...if something goes wrong, we're the first ones to the scene of the crash...so you can bet we're keenly interested in ensuring it's a safe departure and a safe arrival, with a pleasant, safe flight in between. It's in your interest, and ours, too.