RE: Megapixels and Crop % Before Denigration

I am a (relative) beginner so please be patient. Does anyone know if there is

a formula or something similiar that says...for example...if you have a 10 MP camera, then you can crop "x" percent before the picture quality begins to become visibly affected?

Thus, if I could crop ...say...20% in my image editor with a 10 MP camera, perhaps I could crop 25-30% (5-10% more) with an - otherwise - identical 12 MP camera.

I ask this because I keep reading how we only need...some...2-4 MP in a camera to reasonably post to a website (without cropping) but that having more megapixels allows one more cropping latitude. I'm simply looking for a percentage or otherwise quantifying number for various MP (10, 12, 14, 16).

Michelle