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

1. For Campaigns with multiple segments, this setting applies to each segment and not the combined number of recipients from all segments selected.
   
2. This option is not available on Split A/B campaigns.