That sort of thing needs to be figured out before you start inspecting a job. Generally speaking, the percentages of inspection ramp up as failures are being found. ie. If your job specification s say to inspect 10% of the work and if a failure is found, then inspect 25% of the work, failures are still found, 50% of the work gets inspected and then up to 100% inspection until you start finding acceptable work. Job specifications should spell all of this out, if it doesn't send out a request for information and get this question answered before your job starts.

Are you asking how many joints to cut out if a weld fails ??

You would only cut out the ones in which RT showed them unacceptable unless specifications dictated something else.