There is no limit for how many times repair/PWHT of a weld is performed. If your client places a limit, this is their prerogative as long as the repair organizations performing the work understand this specific requirement.

The largest number I've seen is R-9 on a radiographic film.
The radiographer was worried about his charges on one weld.

We state in our purchase order specifications that a welder will be allowed two chances, amenable by the our inspector, at which time a new welder will inherit the weld repair.  

We always inspect prior to PWHT unless the MOC dictates otherwise.  Fortunately most of our PWHT is due to thickness not chemistry except on some alloy piping.     

The only other concern is the number of PWHT cycles may be limited if the material / weld has impact requirements.  See UG 84 & UCS 66