320's Far outnumber all other ML's. 430's were introduced in '99 and were never as popular. Most Mercedes models that are offered with the choice of V6 or V8 engines are the same way. V6 models will sell many times more than the V8 ones will. The ML430 was replaced by the ML500. That will make their individual numbers less, as it divides the V8 models in to two groups. The ML500 was introduced in 2002, with some notable differences. The ML430 has a clutch driven engine fan like the ML320, the ML500 uses an electric fan like the ML55 AMG. Both V8 Model ML's come with more standard equipement than the V6, but anything you can get on a V8 can be optioned on a V6. I often wonder why someone will buy a Sport package ML320 with all the options, then add even more accessories, but not get the V8 model instead. I opted for a 500 with only the sunroof and comfort packages, which was priced very close to a loaded 350. I wanted the V8 more than anything, and a 500 comes with Navi and heated seats standard. The V8 models are usually much more expensive, not only to purchase but the maintenance and labor costs are often higher. Its more difficult to work on V8 models due to space constraints.
The only exception to the V8 vs. V6 thing is the 129 Model SL. There are a lot of 6 cylinder SL320's, but probably more SL500's. It IS a sports car after all.
Hope this helps!