Actually, it depends on the team or to the campaign manager if they going to distribute all token to the one participant. I see some bounty campaign that has only one participant in their signature campaign and I guess the manager will not distribute all allocation for the signature campaign and I guess the rest of the token that allocated to that campaign will be returned to the team. It is totally good if they willing to give all allocated token to the one person who joins in their campaign. Lastly, the decision of the allocated token will still depend on the tear or on the bounty manager.
A project is very rarely able to succeed if only promoted by a participant, I have never seen a bounty campaign that was only filled by a participant. But if that happens I think the team will not give it completely, they will cut the allocation of the token.