How users are counted for Billing ?
All the user roles except Developer is counter for Billing. Developer account has only view permission and cannot report the bug. All other roles such as Admin, Reporter, Client will be able to report the bug and are counted for billing.
What happens if I delete the project inside Disbug ?
Once you delete the project inside Disbug, all the related issues, integration will also be deleted. From the next billing cycle onwards, you wont be charged for the users in that project anymore.