The TIFF is 16 bits per channel per pixel. That is, each pixel comprises three 16-bit values, one each for the red, green and blue channels, for a total of 48 bits. This is true whether the conversion was performed on 12-bit or 14-bit sensor data from the NEF file.
The NEF file contains recorded values from the sensels (the individual light-measuring elements of the camera's image sensor). These can be stored as either 12- or 14-bit values in the NEF. It's not technically accurate to refer to that as "color" depth since raw values haven't been converted to color values yet. That's the function of the raw converter program.