Roll20 is best but you can roll physical dice if you want. Honor system.
OSE wants 6-8 characters:
Size: The ideal size of a group is between 6 and 8 characters—large enough to confront the challenges of the adventure, while not so large as to be disorganised. If not enough PCs are present, the players may wish to hire retainers
Given the simple rules, it is possible for one player to play (run) multiple full characters.
Scheduling-wise, as long as two players (plus one DM) are available, the game can be played. If the players don't have in-world accessible secondary characters or retainers, the game might become very difficult.
Probably 5 or 6 players at once is the max due to online communication overhead.
Some players have mentioned interest but cannot commit to a set recurring time. They will be on-call players that might drop in for some sessions.
Sometimes players may miss a session. When that happens the following rules apply.