Most USA Universities require three. Make sure that your reference letters are on letterhead and signed. We have had questions about what happens when a retired faculty member writes a reference letter. Usually they are able to get from their previous employer a letterhead, although the USA University really just needs some kind of official looking header on the letter. It must be signed.
We sometimes see ‘checklists’ from Indonesian universities. If you have only 3 checklists the chance you will be admitted is smaller. In the best case, you will present 3 original letters, all on letterhead and signed, that are at least a page long describing in detail your work.
Below you will see a sample letter from a reference. Your reference should insert their equivalent of the ABC or the 123. You need THREE OF THESE.
In summary, the reference letter should 1. Be on letterhead. 2. How the reference knows you and for how long. 3. What you have done when they have known you. 4. Their prediction about what you will bring to Graduate Study and your likelihood for success. Why you should be submitted.
Then signed (Typed and with a written signature).