Answer to "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.
Answer to "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.
Answer to "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.