To print multiple invoices please go to the Finances > Invoices page.
Before printing it usually helps to adjust the filtering options by using the fields on the top of the page.
You can filter your invoices by date range, customer, invoice status (like paid, partially paid or pending), etc.
Once you are done setting the filtering fields please use the checkboxes to select which invoices to print.
Once you selected your invoices please click the "Print" link to print all selected invoices: