The manual for DR-8364 series says “For stable video transmission, it is recommended to connect less than 32 network cameras or video encoders to a single Video In port.” What does this mean ?

A DR-8364 series NVR has video input ports which consists of two SFP ports and three RJ45 ports.

Not to place too much load on one RJ45 port, please distribute channels onto switches and connect each switch to each different RJ45 port as the image below shows.

connecting PoE switches

Be aware of that connecting more than one RJ45 port to a same switch can create packet loop, so please separate links from each port.