My guesstimate is that there would have to be at least 5 PTs. 31001 to me is a 'Naval' prototype. The air superiority or multirole J-31 would be slightly different.
YES and NO, especially since actually I don't think that there will be a specialised air superiority J-31, even more since the current 31001 is for sure not a naval prototype, since it lacks the folding wing and tail hook.
IMO aircraft 01 is simply a proof of concept prototype, maybe comparable to the British EAP in relation to the €F or the YF-22 to the serial Raptor. But that will require new decent engines most of all.
I agree with You that in the end if the decision was made to develop this into an operational type there will be more prototypes - IMO even more than 5 - and in the end there will be a true naval multirole version and a land-based multirole version. I don't think that there will me to seperate multirole and air superiority versions even more for both the PLAAF and PLANAF.
As such like Bill Sweetman once said, the J-31 could be in fact the better JSF - maybe to be called a Joint Sino Fighter
- since it can leave the STOVL-requirement aside.
Deino