I wonder:I'd argue the D800 series is a success, but no use if you have a ton of Canon lenses. How many are willing to either convert or go multi-system with Nikon? You need deep pockets to do that. The Sony suffers from the "not enough lenses" problem with any new mount. MP count isn't the only problem people have.
If many here want 36 megapixel, why D800, D810, A7r are not big sales successes?
Personally, before I give an answer to "how many MP", I have to say my answer will rather require a different mindset than is current. I'd say, for an APS-C sensor, I'd like a minimum of 150MP. For the "why" I'd argue not every pixel matters. To me, the bayer pattern sensors have always been a cheat by the manufacturers, but as almost everyone does it, it cancels out. In essence we're not sampling RGB at every single point, and the software has to make it up, sometimes better, sometimes worse. By having a ton of pixels, particularly more than you ever need for output, you get more colour resolution. What about noise you say? Well, noise isn't noise. It will have characteristics. By downsampling to your actual output size, you keep the increased colour resolution and reduce the higher spatial frequency noise. 150MP is roughly what is easily attainable by scaling up compact camera sensors from a few years ago. By today's technology it could be much higher.
Obviously the way we handle this data may need a different strategy also.
The Foveon type sensor doesn't have that problem, but comes with a load of different problems.