How many RAM can i add and who are the manufacturer?
Apple did not solder RAM in those days, so you should be good to go. As far as the manufacturer is concerned, it really doesn't matter. As long as the modules are from a good (and ideally the same) manufacturer (crucial has been good in my experience), match the required specs which you can find at sites such as http://www.appleserialnumberinfo.com by entering your serial number, you'll be good to go.

I would also advise that you use the same sizes of memory modules in both banks. You currently have an imbalance, which can negatively impact performance.