Do you believe the FBI holds the biggest number of bitcoin?
How would we know? How certain are they? We can't tell because nobody/no one would say how many bitcoins he/she has on his/her hands

I would not know for sure since FBI doesn't really disclose those kind of information to the public since it is of a personal investment of FBI. But I don't think the FBI would think of holding on to bitcoins since they don't have a clear use for it. They are enforcers of the law, not som big fish investors.