After you subscribe to a paid plan, you will see the option to activate any of your Demo Apps via the "Activate App" button on the top left of your app card.
Once you activate the app, it will count towards your plan limit.
Are you a team member trying to activate an app in a team?
Only team administrators can activate apps that are within a team. If you need to activate your app, you will have to ask your team admin to log in perform this action