How can I extend my visa exemption in Thailand and what should I know about the process?

Dear experts,

I am Vietnamese, I arrived in Thailand with visa exemption for 60 days (Sept 25 to Nov 11). I have few questions below:

1. I want to apply visa extension, but I don’t know how many days I can extend (30 days or 60 days)? Or do you have any official document about that please share.

2. The extension valid from the date it approved or from the date my visa extemption expired. For example, if I apply for visa extension on Oct 15 (via e-extension), and get approve after 7 days and get the stamp on Oct 22. So the extension (30 days or 60 days) start from Oct 22 or start from Nov 11?

3. This is the first time I do it, so I wonder, is there any situation that the immigration reject the extension?

Thank you!