for example: prize allocation for a campaign (signature) is around 2,000,000 tokens and if the project only has 1 participant in the campaign (signature) and when the project has been completed or not scammed, is it true that the participant will get the sum of all tokens (signatures)?
or maybe not at all, for other reasons?
Some campaign have specific requirements before a type of bounty will be eligible for distribution. If you observe the terms and conditions you will notice something like e.g If the campaign doesn't get enough participants or doesn't meet their minimum limit of accepted participants, a particular percentage of the bounty allocation will be removed etc. And the most common terms which is "The manager has the right to stop/adjust the campaign at any given time".
Therefore, before the signature campaigns even come to an ends, it would be stopped by the campaign manager and most time, fund will be moved to other categories like Twitter or Facebook campaigns. No manager will operating a signature campaign with just 1 member so technically the scenario you highlighted above isn't possible.