Hello,
I would like to mention that this process cannot be automated to autogenerate unique account numbers for all your customers and have it sync to QuickBooks right away. There would be some customization required to assist in doing this. Here is an idea below on how customizaton would be useful for generating an account number to customers.
1. For new customers: The new customer screen can be customized so that when you click save there will be actions that take place to assign a unique account number to every new customer you add and sync that to QuickBooks.
2. For customers that already exist: I would recommend exporting the customers to an Excel sheet and using an Excel formula to asign an account number to the contacts and update your customers back to Method using the import/export tool.
We have consultants who would be happy in assisting you with this type of a project. If you are interested, please reach out at success@method.me