One invoice will be created when using the Create Invoice Apex action in a flow. The Create Invoice Apex action is a custom action that creates an invoice for an order summary. An invoice is a document that requests payment from a customer for an order. An invoice can include one or more fulfillment orders and one or more credit memos. A fulfillment order is a record that represents a part of an order that is fulfilled by a specific location. A credit memo is a record that represents a partial or full refund for an order or a fulfillment order. An order summary can have multiple fulfillment orders and credit memos associated with it, but only one invoice will be created for the entire order summary. References: Order Management Objects, [Create Invoice Apex Action]