Oops. I missed the "only" in the second option when I voted for it.
It depends on the situation.Doesn't allways are one or two best man ,for example in soccer team there are more than two best man ,if team is good.
I voted "two" because I had to choose in order to see the answers, but I think it should be "one man".
For 3 or more think.
what if it's at a wedding ?
Seriously now, I would have said " better " man, but nobody says that ...