The NVR may be able to record up to 12MP streams, but can it record 8 12MP cameras at 30 FPS at the highest quality? I doubt it.
This data sheet:
Hangzhou Hikvision Digital Technology Co. Ltd.
Says it can do 4 channels @ 8MP, or 16 channels @ 1080P. That is Hikvision's capacity listing for the DS-7616NI-I2/8P
It's rated at 160Mbps, so I'm guessing it's capacity is rated at 30 fps.
You have the MP of the camera, but the bit rate will depend on the quality setting and FPS. To make it easy, 1080p at 30 FPS, high quality is about 10Mbps. So, 10x16=160Mbps. To use the system with 16 4MP cameras you would have to lower the bit rate or the FPS in half, or both by 25%. But you would still be at the rated capacity of the NVR.
So, if you are planning a new system, and going to have the NVR already at max capacity, you might consider one with more overhead.
Oh, and WD purple, the largest you can fit, which looks like 6TB x 2.