Now, if I wanted the 16ch model, it also comes with "8 independent 100Mbps PoE Ethernet interfaces". Considering that the 16ch can take 16 cameras, but it only has power for 8 cameras, does this mean we have to attach an external 8-port PoE switch ???
What you end up doing with more than 8 cameras may well be a consequence of how your wiring runs are located.
What you could do is as follows:
Connect as many cameras as you wish into the PoE ports on the back of your NVR.
For the remaining cameras that you want also to power using PoE, get a new PoE switch. The more common, cheaper ones have 10/100 ports, and ideally a Gigabit uplink port. Connect the remaining cameras into the PoE ports on the PoE switch.
Now you have a choice of how to combine the NVR (which will have a gigabit LAN port) and the PoE switch (which may have a gigabit uplink port) into your network.
The traffic from the cameras on the PoE ports on the NVR will stay within the NVR (ignoring for the moment any traffic that results from using Live View or Playback via a PC), so you don't have to accommodate it on your normal LAN.
The traffic from the cameras on your new PoE switch will have to get back to the NVR.
In aggregate this will be approximately (number of cameras on the new PoE switch x around 5-8Mbps each). So if you had say 8 cameras, that's about 50Mbps, which is about as far as you should probably take a 10/100 LAN port, but fine for a gigabit uplink port.
Ideally - you'd have a gigabit core switch, into which you'd connect all the high-traffic items such as the NVR LAN port, the new PoE switch gigabit uplink port, your PC, your NAS etc.
In practice, and assuming you don't have a high-end router with gigabit ports that you'd use as the 'gigabit core' you'd just about get away with connecting the NVR LAN port into the new PoE switch, and the new PoE switch uplink port into the router. But if the new PoE switch has a gigabit uplink port, you could usefully connect that to the NVR LAN port instead, and use a spare port on the new PoE switch to connect to the router.
The weak link here would be the NVR LAN port into the PoE switch (if the uplink port is not gigabit), handling the aggregate camera traffic, but that traffic would not affect the router, which generally would only be handling your internet traffic.
Other combinations are possible.
I hope that makes sense.