You are not enrolled in a campaign until the company approves your campaign application - or when you have approved a campaign invitation. See here when you are enrolled in a campaign.
Make Influence does not set a maximum number of campaigns you can participate in at once. However, it is a requirement that you do not participate in more campaigns than you have the time and desire to promote satisfactorily - both for your own and the companies' sake!
It is expected that you regularly post stories for the campaigns you are enrolled in. We expect that you post stories on at least 3 different days during the campaign period. Make sure you fulfill all requirements for the campaigns you are enrolled in. Read more about it here.
It is also a requirement that you watch these short videos before starting your first campaign collaboration.
So, what are you waiting for? Check out our many exciting campaigns - and apply for all those you have the time and desire to participate in! Good luck :-)