Always Swiss rounds over single elimination for any format or any number of players.

1 – Magic Online

2, 3 – Winston Draft. Increase play time by allowing players losing matches to add another booster to their pool and rebuild their deck.

4 – Team Rochester Draft with one “invisible” person on each team (first players on each team to finish first match plays the invisible player’s deck). The team picks for the invisible player as if it were a normal team rochester draft.

5 – Team Rochester Draft with one “invisible” person.

6 – Team Booster Draft (scroll down) or Team Rochester Draft.

7, 8, 9, 10, 11 – Booster Draft or Rochester Draft.

12 – Here and above you probably want to split into two or more pods and do multiple of the above. Or a big sealed tournament if you want more rounds.

List of the basics of a lot of formats.