Doesn’t have to be PoE cameras. I use wifi cameras too, pretty much any camera with rtsp/onvif would work.
Chances are, a single switch is more cost effective than multiple injectors. But you also need Ethernet routed throughout your house. One alternative is to have the G.hn (powerline) adapter with PoE. This way, you can be both network and power with one plug without wiring your house.
Wi-Fi cameras are not a great idea. Sure, they are convenient, but Wi-Fi is a shared access medium (every device on, say, channel 11, has to “cooperate” with all the other devices about when it can transmit, including devices on neighboring SSIDs) and something that is constantly streaming video (or worse, multiple devices!) is going to quickly consume available bandwidth and offer a poor Wi-Fi experience. (But most people only care about convenience.) Plus, Wi-Fi is easily jammed, which is not great from a security perspective.
Ehh, I have a mix of 4K and 2K cameras, it hasn’t been much of an issue. I run OPNsense with a single EAP670 and there hasn’t been much performance degradation. PoE is definitely ideal but not an option for many, including me since I rent. I think the G.hn plugs are probably my best option for PoE if I really needed it.
Chances are, a single switch is more cost effective than multiple injectors. But you also need Ethernet routed throughout your house. One alternative is to have the G.hn (powerline) adapter with PoE. This way, you can be both network and power with one plug without wiring your house.