In the same way you can put a SUM calculation on an amount field in the portal records, with the SUM on the parent record; you can do a COUNT calculation on a field for the portal records. It would be on the parent record as well.
Wood hiding amongst the trees!
I am glad to be able to RETURN to the Forum for all of the help I have received (Thank you Phil).
I would use Count rather than Sum (though sum can be made to work) and be careful of filtered portals as the count (or sum) will ignore any filtering done on the portal's set of related records.
And you may want to try setting up a layout for printing this data where the portal is sized to have a very large number of rows, but has then been set on the Inspector's Position tab to "slide up" and to "resize enclosing part". This will reduce the size of the portal to just the number of rows needed to display your portal records.
There are also ways, if your data model supports it, to use a list view layout based on the portal table but including fields from the parent table inside header, footer, and/or sub summary layout parts. This layout will not encounter any issues with being unable to print all the records in your portal nor will it leave a lot of unused space on the page if there are only a few records in the portal.
I can't choose a "best answer" because they are both good.
In this instance the single page print out's portal will carry up to 5 transactions, which will cover the vast majority of uses. If the unfiltered record count exceeds 5 then the user is given the option to print a report as an annexure to the single page document.
I will have a play with your suggested method Phil. I will definately have use for that approach with other printouts.