You can supply your onboard account with either gift cards, credit card or cash. If you have a credit card on file (which is easiest) they will put all purchases (including gift cards purchased on board in the shops) onto that credit card. You can add funds on the cruise itself by going to GS and adding gift cards, but they will always use the credit card on file to backup any differences.
So to answer your question, if you purchase gift cards in the gift shop they will go back to your onboard account payment method to pay for those gift cards.
If you purchase a gift card onboard and then turn around and apply that gift card to your OB account, it is ultimately coming from the same source.
I am not sure there is any kind of limit as long as your CC will handle the purchase. So, if you want to buy a couple of 500.00 gift cards, then that will be charged to your credit card OR source of payment you designate on file.
Does this help at all?
