Thank you for reaching out to the Community. The confirmed opt-in function is a great way to ensure contacts wish to receive your campaigns.
In answer to your question, if you upload contacts to use in an email opt-in campaign then those contacts would be included in your subscriber count.
The best route to ensure that contacts have opted into receiving communications from you before you upload those emails into your account is to send the opt-in link outside of Constant Contact. Click here for the instructions. This would also avoid the contacts being included in your active, billable subscriber count.