So far, SpaceX has made and destroyed/launched Starhopper, MK1-MK4, SN1-SN19, BN1-BN3, and BN5. They also have SN20 on the launchpad stacked on BN4. The are stacking SN21-SN22, and SN23-SN24 are under construction. BN7-BN8 also are being constructed. So, what will be the final starship prototype?
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.