I am doing the same thing, possibly up to 4 using an HDMI splitter. I am just learning all this, so I do not speak as an expert and welcome corrections and suggestions.
My Dahua NVR HDMI output is 4K (3840 × 2160) HDMI version 2.0 and 60 Hz. All these things matter when selecting HDMI splitters or Ethernet extenders. Some can do limited downscale of the resolution, some cannot. For 4K, the Ethernet cable needs to be Cat6 and dedicated to the point to point extender devices, not run through your LAN/router. My NVR is PoE so all cameras are connected directly to it. I had hopes of using one of the RoKu apps (IP Camera Viewer) but even though the NVR is ONVIF 2.4 compliant, it seem to be geared towards direct connection to cameras on your LAN and appear not to work through an NVR.
At the moment my cable plan depends on the distance:
1. 25' - will use a 4K capable copper HDMI cable
2. 40' - will use a optical HDMI cable
3. 80' - will use HDMI Ethernet extender
I have not purchased yet, but looking at these devices:
Splitter:
gofanco PRO-HDRSplit4P
HDMI Extender:
gofanco HD20Ext