Mk XIV isn't that different from the Mk IX. In fact, apart from where the engine was manufactured,late model Mk IX and Mk XVI airframes are identical. The only difference is the engine: with a Merlin 66 it is a LF Mk IX and with a Packard Merlin 266 it is a LF Mk XVI
The reason for the differing designation is that the US built Packard Merlin 266 used slightly different measurements from the standard Merlin 66 and so required slightly different tools to work on. To prevent confusion they gave the Mk XVI it's own production number.
For example, Castle-Bromwich produced the majority of IX and XVI airframs. From a batch of 100 Spitfires ordered in October 1943, 65 were manufactured as LF. IX, 10 as HF. IX and 25 as LF. XVI. Exactly the same airframe in each case, just with slightly differing engines.
Actually, when you compare the MK Vb and Mk Vc airframes, they are substantially more different than the Mk IX and Mk XVI airframe.
Mk V production, including conversions and 15 or so identical PR type F, was 6,472.
Mk IX/XVI production, including conversions, was 6,719.
Spitfire IX airframes were the most produced. The Spitfire V was the most produced designation.