On the Contacts step of the Create a Campaign Workflow there is a blank box titled "Set a maximum number of random recipients" (seen below).
You can use this setting to limit the amount of recipients receiving a campaign (as opposed to all contacts in all segments assigned to the campaign).
Notes
This article has been updated to reflect how-to steps in our Air platform. If you're using the legacy platform and need help, please contact support@delivra.com.