There is a lot of value in getting some kind of board of directors or advisory board in place as early as possible. Somebody needs to tell you when you’re messing up.
More importantly, somebody needs to tell you when you’re doing a great job. Without the board life can get very lonely.
Your ability to get the right kind of people to sign up for this is also a good early indicator for your ability to attract employees, customers, investors, partners.
Personally I’d recommend a three people setup at the beginning, minimum two, maximum four.
For a more systematic view on this topic you may want to look at Brad Feld’s book on Startup Boards.