Hi Paul,
Thanks so much for reaching out!
Instead of adjusting the capacity, one thing you can do is to alter the total_sales custom field on the WooCommerce Product page that is associated with the ticket that you are trying to edit. You can change that number to reflect the actual number of attendees that you have, which will then change what you see in your Attendees tab.
With these 10 tickets that were sold offline, were attendees added in manually? If not, I would recommend doing so in order to keep things consistent so that the sales numbers stay accurate.
Let me know if you have any other questions on this topic!
Thanks,
Jaime