Billing Question for Admin Please.

In two parts.

Specifically - on 7 July I was billed for 13 Domains and, whilst I have increased to a total of 10 in the past few days - at that time there were only probably 6 or 7 which makes it appear I have been overcharged.

Generally.   Each time I register a new domain - it shows how many domains I can add on at both zero cost to register and at zero cost to maintain and then shows that "thereafter" !!!!!!! I am billed at 5 per register and 2 per month. Which makes me believe I have a set number of domains available in my legacy account that I can use up to at any time.   Is this correct that I have a certain number?   If so what is that number ?   Then, if that is the case why am I continuously billed for each domain on top of my standard subscription and, indeed that any that I have asked you to recover for me. 

I attach a for instance screen shot of such a notification of which I have a few for safekeeping but it is these zero costs that tempt me to add more to my account - BUT - it appears you keep billing me for individual domains even though I am presuming they are built in. 

   I have raised this point before but now I am determined to get to the bottom of it.