The availability of relatively inexpensive Canon Point & Shoot camera supported by CHDK makes it possible to consturct sophisticated rigs for shooting many images simultaneously ( multicam ). Typical these are for the generation of animated 3D images (sometimes called " bullet time " photography) and may have 50 or more cameras. Another popular application, using only two cameras, is book scanners.
To construct a rig with multiple cameras, there are several complications that you need to consider. Listed below are some of the most complex or difficult issues that must be addressed with tips, hints, and best practices as listed in the CHDK Forum.
Unless your subject is perfectly stationary, you really need the cameras to all take their images at almost exactly the same time. Roughly speaking, this means within about 1 millisecond of each other. Typical software polling schemes via USB are unlikely to achieve better than 100 millisecond syncronizaion (my somewhat informed guess) so this limits what you can have for a subject.
The CHDK USB or A/D channel remote shooting capability, with suitable external switch hardware can hancle this for you. If you go with the USB remote method, you may need a hack to allow you to share the USB port between the triggering and image download functions. The A/D channel trigger scheme is cleaner but only works on camera with a three (or four) terminal Li battery.
link > USB_Shutter_Remote
Unless you are prepared to manually remove the SD cards from every camera after a photoshoot and manually sync the images on your PC, you need some method of downloading from multiple cameras and organizing the images on your PC.
Typically this can be handled via CHDK PTP software (or other variants of that) and multiple USB hubs. There are several forum threads on this and help is available here or the CHDK IRC channel. One approach is to use multiple Raspberry Pi's to split the load if you have a lot of cameras.
Trying to manage charging batteries for multiple cameras and keeping them all running quickly becomes impossible.
No surprise here - you are going to need external power supplies. Amazon & eBay sell inexpensive supplies for pretty much every Canon camera model ($10-$20 ea) made in China. The quality of those can vary a lot. Plan on purchasing quite a few spares. The official Canon supplies are very good but cost around $60 ea. You can also "roll your own" supplies using cheap DC/DC switching power supply modules and bulk power supplies. Be careful about net power draw, grounding and wiring if you go this route.
Turning on all the cameras can be a big job, and getting them all into the same configuration can be a challenge too.
There is not really an easy way to turn all the camera's on at once under network/usb/software control. Unless you want to manually press every power button, you really need to user a device to physically clamp the power button down on each camera and then start them by supplying external power (see item 3 above). After that, you can have a CHDK start-up script on each camera set them all to the same settings. You can also use CHDK to prevent the cameras from retracting the lens automatically after a period of inactivity.