12MP but I believe it is using the same ~30% larger image sensor found in the iPhone Xs wide angle camera. That Paired with the A12 Neural Engine & ISP, the camera on the Xr should out perform the 6s in every way. I would say it will likely even outclass the 7 in just about everything except optical zoom.
The XR should beat every previous iPhone camera (including the X and 8 Plus) in everything but optical zoom, although it remains to be seen if Apple can do portrait mode well with a single lens or if older dual lens camera will still beat the XR when it comes to the quality of portrait mode photos. Either way with it's larger sensor, optical image stabilization and more advanced ISP (Image Signal Processor in the A12 chip) it will definitely take better photos than the 6s (although admittedly you may not notice much difference in brightly lit outdoor photos, but in low light or "challenging" lighting the XR should take vastly superior photos every time).