SpaceX is basing its plan on the idea that its Merlin 1D engines have a lifecycle of 40, thus a Falcon 9 first stage could in theory be reused that number of times. The margins built into the rocket allow for the extra weight involved in using landing legs and the extra fuel that will be needed to execute a powered descent. These margins will still allow them to launch substantial payloads to low Earth orbit and a geo transfer orbit.
However, citing their own experience in trying to reuse engines, both NASA and the CNES have suggested that both the technical challenges and the economics mitigate against SpaceX being able to reuse all or part of their rockets. NASA found that it was not worth trying to reuse the space shuttle main engines after every flight without extensive refurbishment. The CNES studied reusing its Ariane 5 solid rocket boosters liquid fueled and reusable but soon scrapped the idea. Safety issues surrounding flyback boosters were also cites as a show stopper