Can you then feed them all to YouTube without switching them also?
I'm not sure what you mean there.
If you're streaming each one separately, fullscreen, to different YouTube videos, then you'll probably need multiple instances of OBS for that.
Then you might have difficulty encoding all of them at the same time on the same computer (multiple physical machines can solve this, with one camera per machine), and in getting all of those separate streams up your internet hose at the same time (multiple machines will not fix that).
If you're putting all of them into a single frame to show together as a single video, then you can...well..."just do that," as most of us tend to think here, since it's one of the more basic things to do. Add a source, connect it to a camera, add another source in the same scene, connect it to another camera, etc. Then reduce the size and drag them to fit.
Right-click -> Transform -> Edit Transform might help, as you can type the numbers in and make them fit exactly.
Then you have only one encoding and one stream to upload, using one instance of OBS on one computer.