How to make a cell multiple form a different cell , but only once a third cell reaches specific number

I have to keep track of different items that are being sold, the first 5 times the client purchase the it doesn't count towards a (punch card) after that every time they purchase it again counts.but I don't know how to do this with out making my inventory total negative or wrong if one person is hasn't bought more than 5 and other has bought 20+. Because costumers are only allowed limited quantity per quarter (50items) that way one person that buy all. I hope this is not confusing. Thanks for the help.