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I'd be glad to help you create a customer number in QuickBooks Online, @piliguamishcampi.
You can assign account numbers to your clients in two ways. First is internal, where you can sort customers numerically and the account number appears in front of the customer's name. Here's how:
The second option is external facing. The account number appears with the customer's name on the invoices.
I've also added this article that can guide you update your customer's profile in QuickBooks Online: Edit customer information.
That should do it. I'd be glad to provide additional help if you need some clarifications with the process. Have a good one