If an office building has 6 security cameras, does a hacker have to concentrate on and Command them one at a time?

Or does accessing one give control of the entire "camera system"?

The simple answer is it depends on the security system.

If the security system has all the cameras in a singular node and a single subscription list, then it would be possible to subscribe each camera like a drone and give them identical instructions. If you want each camera to do something different, then it would be independent commands.

If the cameras are in different nodes (each wing or office has it's own node) then the hacker would have to exploit each one separately and focus his concentration on a particular node and give each command separately.