Hello,

Step 1 - Make sure you are creating the main customer (you can keep the customer type as Company)

Step 2 - Create all the branches as Contacts of that main customer (same as customer but the type will be individual and select the Company as main customer name)

Step 3 - Create a Service product

Step 4 - Create multiple Sales Order with the service you provide and along with the price

Step 5 - At the end of the month, select all these Sales Order (using the select all option in the list view) and from Action > Create Invoice.

So, you will have only one invoice for the whole sales you have done for that month. 

In Odoo, you can maintain only one Receivable or Payable Account and you can use the same account for all the customers or vendors.

You can check the below blog to understand more about the accounting setup.

https://www.cybrosys.com/blog/how-to-setup-payable-receivable-accounts-in-odoo-15#:~:text=Below%20the%20Accounting%20tab%2C%20you,of%20accounts%20for%20specific%20customers.