Hi there, @Bogierich.

I'm here to help guide you delete old customers in QuickBooks Desktop (QBDT).

If you're using QuickBooks Pro/Premier, yes, the maximum number of total names(employees, customers, vendors, and other names combined) is 14, 500. To enter/add a new customer in QBDT, please ensure not to exceed the limit set by QuickBooks. In your case, you really need to delete old customers or inactive customers. Let me help guide you on how.

1. Click on the Customers menu.
2. Select on Customer Center.
3. Right-click on the customer you want to delete.
4. Click on Delete Customer:Job.

Do the same steps for the rest of the customer you want to delete. Once completed, you can now add a new customer.

1. Go to the Customers menu.
2. Select Customer Center.
3. Click on New Customer & Job.
4. Select New Customer.
5. Fill in the necessary information.
6. Click OK.

To learn more about the maximum number of lists entries in QBDT, you may check this article: Maximum number of list entries (list limits and custom fields).

You can also check this article for your future reference: Help articles for QuickBooks Desktop.

Stay posted if you have any other questions about adding and deleting customers in QBDT, I'll be always here to help you. Have a great day ahead!