Not without changing your layout.
You can size the portal to show all 20 lines and use the sliding option in the Inspector to slide the portal up and resize the enclosing part to remove the unused space. Note, however, that if you get 21 portal records, the 21st record will not be printed unless you make your portal even taller to show even more rows.
You can also base a layout on the portal table, putting fields from the parent record in the header or a sub summary part with the portal table data in the body. In list view, the resulting report can print out with any number of portal records--which makes this the most flexible approach. The invoices starter solution prints invoices in this manner if you want to see an example of this approach.
how do i find the invoices starter solution mentioned above?