The portal will only show (in print) the lines that show on the monitor.
The typical way to get a "full disclosure printing" through the portal is to create a separate print layout with a portal in it with more lines (like 100). Then use sliding (Set sliding/Printing) to slide any text below the portal up to fill in any gaps.
If you have 20 prtal records, it will print 20/100 then slide the footer info up.
Pick a number which you know you'll never exceed, and use that number as your # of portal rows. This can be a real pain when viewing the monitor...so make this layout simply a "Print layout" and continue using your 15 line portal for normal day to day.
The other option is to not print from a portal at all. Take a look at how the invoices starter solution prints an invoice. While the invoice line items are entered in a portal, the invoice is printed from a separate layout based on the lineitems (portal) table instead of the invoice. The Invoice fields are added to this layout in header, footer and trailing summary layout parts while the line items fields are placed in the body. The layout is set up as list view.
With this approach any number of portal records may be printed, no matter how large.