The Cron job that when triggered checks all your campaigns if there is any campaign that is eligible for processing. and if there is one or more it process just one and the next time it gets triggered it process another one.
so at any time, there will be one campaign processed.
If you want to control how many posts posted to your site from all campaigns at a specific time frame, you can set the post status to Draft and use this free plugin to post Draft posts at specific times
[Update]: Draft scheduler development stopped while you can download it from here https://downloads.wordpress.org/plugin/drafts-scheduler.1.9.3.zip