Continuous Forms Record Count

I've only recently gotten to know about continuous forms and their use a bit. So for example, I used a maximum number of pre-defined (6) visits. I need to change that still, but during the development of the sales forms I already ran into a problem.

At first I was planning to have a certain maximum number of items to have on a form and create an invoice from that. But of course that would limit the number of items on one invoice which isn't good. So, now I have a continous form which adds a line containing productname, price, nr of that item etc. anytime needed.

In the previous design idea it would have been easy to calculate the total amount of money at the end of the list since the list was predefined. So I could just say invoiceamount = itemID1amount + itemID2amount + itemID3amout etc.
With the "new" design however that is impossible to do since the number of objects is unknown. So how do I create a piece of code which does that? In the continous form I have a SaleID which keeps track of which items belong to a certain Sale and a TotalItemsID autonumber which keeps track of the number of lines (items) which belong to that specific Sale.

So I need to count the total number of TotalItemsID's within a certain SaleID and then have a piece of code which creates a sum of that counted list of items.

I'm new at this, I've found the function Dcount() should be used to accomplish this, but the rest, as of yet, remains a mistery to me.

How to do this?