It depends. Very good friend of mine got about 80 000$ for 5.5 months bounty compaigns. He worked day by day on 50 compaigns at the same time.
it's amazing that your friend is smart enough to follow 50 campaigns in one account, if it's true how can one account just be used for one campaign and one signature?
Wow 50 campaigns at a time, that's insane ! It would have taken complete days to achieve this. Was he using a bot ?
Also, it would have become quite difficult to keep track of all the work, was he enrolled in one of the automatic bounty platforms ?
Answering to your question, it is not possible to join more than one signature campaign at a time from one account. If anyone is using his alt accounts for campaigns, then it is violation to the campaign rules. However, one can enroll in as much social campaign as he wants.
You are correct, one can participate in a number of campaigns such as content bounties that might require you to just do one article so 50 is quite doable, I once did 30 articles in one month, however quality of the content might be affected due to having to write on something different every day.