So far, Spacex has made and destroyed/launched Starhopper,MK1-MK4, and SN1-SN10. They also have SN11 on the launchpad and BN1,BN2, and SN15-SN20 under construction. What will be the final Starship Prototype Spacex makes?
The one that achieves all the goals SpaceX has set for the prototype program.
We don't know which goals those are, they have not been publicly disclosed. Even if we did know, we wouldn't know which prototype achieves those goals, because the whole point of prototypes is that you don't know whether they will work – that's why you build them in the first place.
Since we don't know what the goals of the prototype program are, and not even SpaceX knows how many prototypes they will need, literally nobody knows the answer to the question.
SpaceX started building SN12–14, but then later realized it doesn't make sense to test them and scrapped them again. SpaceX also built Mk I for flight testing, but that didn't work out either. Clearly, not even SpaceX knows what prototypes they need.