How do i assign mandatory customer numbers to identify the customer when creating customer master?

Thanks for checking in with us, akrishnanca-outl.

In QuickBooks Online (QBO), we can assign account numbers to your customers that can be internal where you sort customers numerically. We can also set it up as an external-facing that will appear on customer invoices. Here's how=

1. Click Sales on the left menu. 
2. Select Customers/Clients.
3. Choose a customer to open the customer page, then tap Edit.
4. In the Display name as field, enter the account number before the customer name.
5. Press Save.

The account number now appears in front of the customer's name on internal lists and screens. For customer-facing numbering, just follow the steps below: 

1. Go back to the Sales menu. 
2. Select Customers/Clients.
3. Choose a customer to open the customer page, then choose Edit.
4. Add the account number on the Use display name field. To see the account number reflected directly under the customer's name use the first line of the Street field.
5. Press Save.

then, the account number now appears with the customer's name on invoices and other items. Feel free to visit our Manage Customers and income page for more insights about managing your company's income and sales transactions.

I'd like to know how you get on after trying the steps, as I want to ensure this is resolved for you.  Just reply to this post and I'll get back to you. Have a great weekend.