I'll be more than happy to help you obtain the information you're looking for. Can you clarify what exactly you mean by client number? QuickBooks Online companies, clients and firms alike, do have an identification number used for referencing to support. Finding that ID number is a breeze, just follow these steps:
Finding your client's Company ID
From your QuickBooks Online Accounting firm, navigate to the client company from the dropdown (▼) menu at the top.
While on the client's Dashboard, press CTRL+ALT+? (the question mark button on your keyboard).
The Company ID will appear in a pop-up in the center of the screen. If you'd like, make a personal note of it for future reference.
I'm including a screenshot displaying what this pop-up looks like in our sample company:
This enables you to access the company ID for whichever client you need. If there's something else you need to find, please don't hesitate to let me know, I'm always available here for all of your QuickBooks questions. Thanks for dropping in, I'll keep an eye out for your response.