Hi Simon,
It's not possible to limit the no of people in a campaign . However you can change the Success metric advanced options to give you the ability to define custom behavior for both content viewing and counting when visiting a success metric. Please see the following for Information
Also, , you can change the percentage of visitors to be included in the campaign or test defines the total percentage of your qualifying visitors who will see one of the experiences of this campaign. The remaining percentage of users will see the default content for the mbox. Please see link
If you are looking for Transitioning from Target Classic to Target Standard , Please see the following documentation
Thanks & Regards
Parit Mittal