The number of megapixels of a camera is determined by the resolution of the camera's sensor, which is part of the camera body. It cannot be altered by a lens.

Does lens increase image quality?

The camera lens has a greater impact on the photo quality than for example megapixels because a camera lens has a direct effect on the background blur, sharpness, level of detail, depth of field and these are just some of the more important parameters.

Is 12MP better than 48MP?

As we mentioned before, 12MP cameras usually allow for larger individual pixels than super high-resolution sensors. The larger the pixel size, the more light each pixel can capture. A 12MP half-inch sensor would produce far cleaner low light shots than a 48MP half-inch sensor, given that every other variable is equal.