Officially, we can say 1 IPC linked from the camera.

Power
PoE, IEEE 802.3at(Class 4), 25.5W
(DC OUT : 42.5-57V=, 0.3A MAX)

DC OUT is extended port.
42.5V x 0.3A = 12.75W (min)
57V x 0.3A = 17.1W(Max)

So, at least DC OUT (Extender port) can provide 12.75W~17.1W

Basically, DC-D4517 series consumes about 7W.
12.75W – 7W = 5.75W. thus it’s not enough to support additional third camera.