I only do one bounty and do justice to that particular company. I believe in honesty and integrity. I do not participate in multiple bounties at the same time and pay dedicated attention to one particular bounty. I could do that, but I have high morals and very ethical person. Also, I implore you to do the same. It's good to partake in only one bounty as they are paying money for your service so you, in turn, should do them justice. So according to my humble opinion, one should only partake into one bounty at one point of time. It is never good to be greedy. Give your all to the current bounty.
Naturally, in a signature campaign, you can participate in only one. You can not take several signatures at once.
But who forbids you to perform several bounty campaigns of various projects on social networks? This is not prohibited by the rules.