That's a matter of some debate. Somewhere between 6 and 12, I believe, is the range.

SpaceX plans to stretch the second stage to increase fuel carrying capacity for tanker flights, so the actual number is TBD until both the revised Starship and its engines (Raptor 3) are built and tested.

It depends on how many Starships you can launch simultaneously, because the time the fuel depot spends in orbit determines how much fuel is lost to boil-off. If you have a serial launch architecture, assuming reusability, refuling and relaunching, then the upper bound is around 20 launches. Even with expendable Starships, this is still cheaper than SLS, but not by much... This is why reusability is critical to the mission.