I'm doing something like that in a building. I'm using a couple different variations of Foscam IP cameras (about $80 each). They all ftp images based on either elapsed time or motion detection to a Network attached server.
The Foscam cameras all run off of 5 volts, so a 12-5 volt converter could run them. They draw about 4 watts a piece during the day. I have never measured them at night when the IR illuminators are on.
The Network attached server (WDSHARESPACE) runs of an external power adapter, so I am pretty sure something like that could run off DC as well. The drives go to sleep after a minute of activity and the power consumption drops quite a bit. There is a place in the server to plug in a USB memory stick. I ftp all my images to the memory stick so that I don't have to wake up the hard drives all the time. A PC transfers all the saved images from the USB stick to the RAID hard drives a couple times a day.
I am sure you could do the same thing with USB cameras. Based on my limited experience with USB cameras however, they take a fair amount of processing power for each camera. I'm not sure what would happen if you plugged so many into a PC. Also, you will run into issues sorting the cameras out if they are all the same kind. With Ethernet cameras, each camera has it's own I address. With USB cameras it isn't so clean. It is hard to sort out each camera from each other camera. I tried it once many years ago with just 2 cameras on a Windows 2000 machine and had many headaches. I wound up replacing one of the cameras with a different one that had a different model number and that stopped them from fighting with each other for drivers and such.
If you do try to send lots of video over a cellular wireless connection, be careful as to the encoding method. For instance, the Foscams all come in two different Variations... either MJPEG or H.264. While MJPEG is a better format as far as picture quality goes. the bandwidth requirements are maybe 20 to 50X greater. No way you are going to stream multiple MJPEG feeds at once over cellular wireless.