Frequently asked questions How much cannabis can I possess in Montana with an MMJ Card? In Montana, you can possess the following amounts with a valid MMJ Card: – with a named provider, up to 1 ounce of usable marijuana. – if the cardholder has not named a provider, the cardholder may possess up to 4 mature plants, 4 seedlings and the amount of usable marijuana allowed by the department by rule (1 oz. usable cannabis).