How many cofounders on board of directors?

I'm doing post-incorporation setup right now. We have four cofounders but haven't raised yet. Expect one spot at least on the board for the CEO, but there are still three others.

I see DoorDash has all three founders on the board. But I've also seen advice that too many founders is seen as a negative by investors.

Also, if we haven't incorporated yet, should we? We were thinking of doing it with the investor's help only once we raised funding.