If you get a 16 and never load it up at least you will have more encoding power. Some of the NVRs have crap FPS on higher resolutions or a few 4K cams and all the sudden you are at 80% cpu capacity.
Had a 8, gave to a friend to get started. Then bought a 16. Had the cpu load problem with 5-6 high res cams. Sold that one and bought a NVR6a08-32-4ks2. I have grown to 18 cams with my last cam, a 25x zoom starlight ptz with auto tracking.
If you get a 16 and never load it up at least you will have more encoding power. Some of the NVRs have crap FPS on higher resolutions or a few 4K cams and all the sudden you are at 80% cpu capacity.
Get the 32. Just when you think, "Ahhh, 16 is enough..." And then it isn't. Buy the non-PoE version and use your own switch(es) since the 32 channel PoE version still only has 16 ports.