Supported starting with API 14 (ICS), getMaxNumDetectedFaces returns the max length of the array that contains the results of camera face detection after it is started with startFaceDetection. If the value returned is 0, it indicates that the device does not support this method of face detection. In this case, face detection must be implemented by some other means (e.g., using FaceDetector available since API 1).
I checked the value returned by getMaxNumDetectedFaces for both front and back cameras in a few devices:
- Nexus 4: returns 2 for both cameras.
- Nexus 5: returns a very large number (5308421) for both cameras.
- Nexus 7: returns 0 for both cameras.
- Nexus 10: returns 16 for both cameras
- Sony Xperia Z1: returns 5 for both cameras.
In summary, OEM support for camera face detection is hit and miss.