2000 Ford Explorer timing

I have a 2000 ford explorer sport and one set my timing chains went out so I had someone replace them but they messed on the timing and my whole engine which had 118,000 miles went out so I then replaced it with one that had 80,000 miles but someone told me that there was another set of timing chains right next to the fire wall that are not connected to the engine is this true? I am worried that they might go out next. so how many sets of timing chains are there?