Separate fields for each item is not a good idea. These should be separate records all related to the same invoice record. The Invoices starter solution is an example of this method. As is this much simpler file created by Comment: http://fmforums.com/forum/showpost.php?post/309136/
Both files use a portal with a scroll bar so that you can add as many items as you wish, but then use a script and a different layout for printing out the invoice so that all items will be listed no matter how many are added to a given invoice.
thanks for the fast response, I will check it out and see if it makes sense to me!