You can set a limit to the number of subscribers in your campaign.
Go to 'General settings' by clicking the cogwheel icon:
You can set your limit on either max subscribers or active subscribers.
Max subscribers count every unique signup, including those who have unsubscribed.
Active subscribers count only those who are still active, excluding unsubscribes.
Test users does not count towards any of the limits.
When you have set up your limit, you can follow the number of signups against the limit on the startpage: