Hola Eboy -
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Yes, there are "spare airplanes" available.
But depends where. As an example, for us, a South American airline, we have a 747-200 and a 737-200 who are available as "spare" on short notice, should one of our other airplanes have a technical problem. But, these two spare airplanes are located at our base, in South America.
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So, if you are going to travel to Europe, say, on a 747-400, and the airplane found to have a major problem, the "spare 747-200" might fill-in, but that will be with some amount of delays. In a typical scenario, the "spare" will be ready and departing some 4 hours later, than originally scheduled.
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Spare airplanes, say in Europe for us...? - There are NONE. In that case, we might try to book our stranded passengers on other airlines, or in a worst case scenario, even charter an airplane from a European carrier. Or put 350 passengers in hotels until we fix the machine...
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Not too long ago (I fly the 747-200s), we had to rush our "spare 747-200" to Sao Paulo GRU airport (empty) to pick-up 300 stranded passengers of a major European carrier, and fly them (very unhappy people) to Europe, where they arrived with some 12 hours delay. Even though our cabin staff had cheerful smiles, these passengers were not exactly ready to thank us for being "late" to assist them to get to their destination.
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Upon arrival in Europe, a few of these passengers even asked to speak to the captain, to express their displeasure in being late. Not our fault, we did our best to help. They were not OUR passengers. And sorry, not many of our spare flight attendants did bark the language of these passengers, they all can speak English well, and sometimes "only" 2 or 3 other languages...
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No wonder that many pilots love to fly cargo. Little boxes do not insult the crews.
Happy contrails.