Hi there, johnskim9.
At the moment, the invoice number field will work best for nine digits. More than that, you'll get 1 on your next entry. This happens when creating sales receipts, estimates, and sales orders as well. I'm sending feedback to our engineers about this behavior.
In the meantime, you can consider entering the transaction numbers manually. I case you'll enter a duplicate one, you'll get a prompt about it.
Let me know if you have other questions in mind.