The sensor has 48 (or 50) photosites (pixels), but the Quad Bayer filter in front of it turns it into a 12MP sensor, so to speak. The filter covers four pixels behind each color square instead of just one.
"So, really these 48MP Quad Bayer sensors can’t offer much more detail than a 12MP sensor. Sensor and phone makers alike will tell you that smarter demosaicing algorithms can capture more detail, but our experience is that the gain is small – if there’s a gain to be had at all."
Read here:
Marketing departments are trying to sell you on the wrong thing - resolution isn't why these sensors are good, it's the better image processing.
www.gsmarena.com