In most cases, you will have a single invoice on each project - the primary invoice. But if you need to add more than one invoice, you can! Here's how:
In the Invoices tab of the project, click on the Invoice List sub tab.
Click Add in the upper right corner.
Build your invoice line items as usual.
When you are ready to send this invoice, click the Send dropdown and select via Email.
Edit the email if needed and click Send.
Now you can go back to the Invoice List tab to view all of the invoices in the project: