How to simultaneously trigger many cameras together?

There are several ways, depending on the types of cameras you are using. Hopefully, they're all the same which will make the job a lot easier. If it has electronic shutter control, that they can be triggered by electronic means. If not, then by mechanical means, such as relays, solenoids or cables. You didn't say how many cameras or give much additional info, so that's about the best I can do for now.