How to derive running Invoice number in AR, in Project billing

Dear Experts,

I have some critical requirement from my customer.

AR module is already live and been used since couple of years.

The AR invoice number in automatic and default value is 10001.

By now, in AR, 17575 is latest next possible invoice number, in that OU1.

Now, that, we are implementing Project billing, and customer wants the project invoice also should have/generate the AR invoice running number. After this project billing live, we do expect, some invoices can alone be created in AR and some can be from project. Meaning that, it is still expected to perform non-project and project related AR invoices in Oracle by customer.