How many cartridges can I buy at once?

You can purchase 5 grams per day some cartridges are half gram and some are grams

Might think about calling the company. They will have more reliable information than online strangers. You can also see how much is contained in a cartridge and look up how much you can legally buy at once. Also be aware that the amount you can hold on your person is possibly different from the amount you can have at home. I don't know the facts, just giving my $.02.