I would have to write it to give a hint on how to do this ...

You need to look at the
/includes/modules/shipping/item.php

Shipping module ... and it might be a good idea to clone that so it does not get overwritten on an upgrade ...

And then find where it does the quote in:
PHP Code:
    function quote($method = '') { 
And alter how that calculation is done based on how many items there are ...

Might be easier to just use a Handling charge and a per item charge, as you are not adapt at coding php ...