Any resource on Forms and PHP.
You can read about: $_POST in any php resource. If you own Ecart you can probably just try it and be successful. Just think about what you want to do and do it.
1) Add a list to the page to show the options of the number of rows to display
2) Set a session variable with a default value of the number of rows you want to display by default. (you can use the server behavior that comes with eCart for setting session variables default value when they aren't set as the trigger)
3) Update the session variable from the form post (again use the eCart server behavior in the general category to set the session variable from the form element, using the form element as the trigger)
4) Set the dynamic attribute of the list so that it selects the current value of the session variable as the number of rows so that it will be accurate
5) Find the reference to the number of rows in the code... maybe update the number of rows in the server behavior to "1234" so that it will be easy to find in the code. Wherever it is referenced, replace it with a reference to the session variable that we created and tied to the list in steps 1-4 above