Thanks for getting in touch with the Community, crisstax.
You can do it either way, depending on how you prefer to record your transactions. For example, you can create one invoice, with the retainer fee and any other amounts they owe, then have your customer make a partial payment for their retainer, and have them make another payment at a later time for the other amount(s).
You also have the option of creating one invoice for your retainer fee, then receiving payment on that. Afterwards, at a later time, you can create a second invoice for them with their other owed amounts on it, and have them pay that as well.
I've also included a detailed resource about working with invoices which may come in handy moving forward: Create & manage invoices
Please don't hesitate to send a reply if there's any additional questions. Have a great day!