Slightly OT as phrased, but it's perfectly reasonable to want to know the total information content in the medium you're using. "pixels" might be a discrete/digital term, but they're no different from linepairs-squared except for a scaling factor.
I get about 95MP (4000dpi) from a Nikon LS-8000, RZ67 and Velvia/Provia/Pan-F/Acros but 90% of the time, the rest of the system (optics, tripod, subject motion) is the limiting factor. 50MP is about as much as you can hope for with a practical shot and if you're even slightly sloppy or like shooting wide open, you can expect that to be more like 10-20MP at most. The same applies to any other capture medium like a digital back though, and btw they're past 80MP now in 645 if you have $25k.
I generally shoot with my RZ instead of a DSLR for things that matter (and where I have full control of the scene) because, when careful, I can get at least twice the resolution as from the best (21-25MP) DSLRs. However, the DSLRs will work well at higher ISO (e.g. 400 easily, some fairly cleanly but with reduced DR at 3200) and larger relative apertures (no f/1.4 lenses for 6x7!) and can therefore capture things with faster shutter speeds, less care and in low light much more effectively. If you go to ISO400 in 6x7, most of your resolution advantage is lost. Of course, there's little need for high ISO in stock photography since everything tends to be flash-lit.
I would never want to shoot stock at 6x7 because the market tends to want a bazillion different subtle variations on things, 95% of which will be completely ignored and therefore successful stock shooters each have many tens of thousands of images for sale concurrently. Your per-frame costs (film and E-6 soup) and scanning time/cost (a few minutes or a few dollars per frame) will kill you. Consider also that while you can do twice as good as a DSLR on a good day, the DSLR is quite good enough for advertising (magazine double-spread) purposes so no one is really going to care that your files are a bit bigger. And the per-frame sell-prices are depressing.
Commissioned commercial shoots are a different matter.