As previously stated this is a goal specific question. If there is a need for a big change in flexibility or mobility then more than one set is most likely needed. If it is not the main focus and the client just needs maintenance then one will suffice. With that being said this is also a joint by joint case as I personally have certain ROM’s that require more on one side of my body than the other. For instance my right hip flexors requires 3-5 sets to restore length wheras my left hip flexors only require 1-3 sets.