If you create a customer in Pool Brain and then see a random 2 digit number appear after their display name, this is because the same exact display name already exists for a customer in Quickbooks Online. QBO requires that every customer display name must be unique and it is not something we can control.
How to resolve a duplicate customer issue
If a customer "Richard Hendricks" already exists in QBO and you create a customer named "Richard Hendricks" in Pool Brain, QBO will add a random 2 digit number to the customer in order to save it in their system (since names can't be identical in their system). Then QBO syncs the name change back to Pool Brain so they match in both places. So in this case, you'd have your original customer named "Richard Hendricks" in QBO and another customer named "Richard Hendricks 27" in QBO. You would only have a customer named "Richard Hendricks 27" in Pool Brain (the one you just added which QBO edited to have "27" at the end).
If you DO want the customer with the same name in QBO to sync/link to the customer you created in Pool Brain (same customer)...
Delete the newly added customer in Pool Brain
(in the example above, it would be "Richard Hendricks 27")
Delete the newly added customer in QBO
(in the example above, it would be "Richard Hendricks 27"
Get the customer that already exists in QBO to import into Pool Brain instead of creating the customer in Pool Brain manually.
Check this article to troubleshoot why the customer in QBO isn't syncing to Pool Brain and resolve the issue.
If you DON'T want the customer in QBO with the same name to sync/link to the customer you created in Pool Brain (different customer)...
Change the customer display name in either program to anything other than a name that already exists in QBO. Using the example at the top of the page, you would change "Richard Hendricks 27" to "Richard J Hendricks" or "Rich Hendricks", etc. You can use any name that is acceptable to you and doesn't match a display name that already exists.
To change a customer display name in Pool Brain, go to a customer page and hover over the name at the top of the page until you see blue "Edit" text appear. Click "Edit" to edit the display name. This will only edit the display name and not the contact name.