Forty-two, which is the number every brochure leads with. Your day trip will see a handful of them and that is plenty.
Only with your own boat, and the park and marine rules still apply to you. Realistically, nearly everyone books an organised trip out of Koh Samui.
No, and people muddle these two constantly. Ang Thong is a marine park northwest of Koh Samui that you visit for a day. Koh Tao is one island further north where travellers go to dive and to stay.
There is a yearly closure so the place can recover, but the dates shift and operators do not all stop on the same day. Look at live availability and current park notices. An article from two seasons ago will send you wrong.