I have 5 12-2 wires coming into a junction box, so 11 wires at 2.25 cubic inches equals 24.75 cubic inches. One of those wires is coming from the breaker panel, the others are going out to power switches in 4 other rooms. What I don't know is if I need to add more cubic inches for devices in that junction box because I don't know what is considered a device.

Ok, I think I found the answer to my question in the NEC.

A device would be a switch, outlet or light fixture?

If someone would confirm this for me I would appreciate it.

Need to allow two times 2.25 for a switch or receptacle in that box.

A fixture is not a device, and only needs to be counted if there is a supporting mechanical device inside the box to mount the fixture, which most do not need nowadays.