The claim is for one of the engines, but on average, how often will they have to replace the worn-out Merlin rocket engines before reusing the booster first stage?
Ah yes. That is for the booster stage.
For the Merlin engine itself, it just says 10+
But bear in mind that the Merlin uses Kerosene fuel
For a clean-sheet reusable engine, Methane is a better choice