Number of pixels is unrelated to diminishing returns (as long as total sensor size is inside plan region). Size of pixels is what matters for sampling. What size of pixels is sampled correctly, depends alot on the optics you use on a case by case basis. The reason why you hear people say "above x MP you get diminishing returns" is generally when they are talking about a set total sensor size, where more pixels = smaller pixels, and smaller pixels are where diminishing returns happen. Its a dangerous misconception, because not all sensors are the same size!
With that said, there's some other information to go over.
For OSC (one shot color), pixels should be treated like they are 2x larger, due to the Bayer pattern.
notice how blue and red are spread out? that means you have to sample for pixels twice the size, to sample red and blue not just green correctly.
To calculate the resolution, use .61 x wavelength / numerical aperture. For wavelength, 550nm is used as a standard green value, but ideally you would want to be properly sampled even into blue.
To calculate size each Bayer pattern occupies use pixel size * 2 / total combined magnification. The total magnification would be the mag of the objective, multiplied by any other optics.