Respuesta a "Can one change the Ram on the MacBook Pro 13'' Retina?"The default can be 4GB, 8GB or 16GB but it's installed at the factory. It can't be upgraded.
Respuesta a "How many OSs can I run?"Well technically, you can run as many as you can fit. Technically, I'm afraid you can't. Would it be possible to have Mac OS X, Mac OS 9, Mac OS 8 and so on on separate partitions? No. 10.4 is the earliest that your eMac will run.
Respuesta a "How many OSs can I run?"You can't normally use an OS that pre-dates the computer. For your model that's OS X 10.4. It won't boot into OS 9 or anything earlier.