A megapixel rating is the total pixel count of a camera sensor, roughly one million pixels per megapixel, so 2 megapixels is 1080p. Megapixels set how much of a scene can be captured, not how much detail reaches one face or plate.
More megapixels sounds like more security. Used across a wide view, they often deliver less detail on the subject than a lower-resolution camera with a longer lens.
Pixels spread across a scene
The same 8 megapixels over a whole parking lot puts fewer pixels on each car than 2 megapixels on one aisle. Coverage and identification are different jobs.
Costs that follow
Higher resolution means more bandwidth, more storage and often worse low-light sensitivity, since each pixel on the sensor is smaller. Size the whole chain.