It's more about what features and specifications your NVR would require: if you are going to add "only" 3 cams, a "simple" NVR4xxx would suffice, but if you're aiming high (with all smart codecs, 320Mbps bandwidth, up to 12MP preview/playback), you're up for the NVR5xxx series, called not for nothing the "professional" series.
If you have picked the series, then you need to think whether (or not) you want onboard POE ports (which eliminates the use of a separate POE switch).
To buy a NVR based on how many cams you will use, if within 8 cams, use NVR5208-8P-4KS2 is fine, if budget is limited NVR4108-8P-4KS2,NVR4208-8P-4KS2 if within 16 cams, use NVR5216-16P-4KS2E.if budget is limited use NVR4216-16P-4KS2.