Usually you can pack about 14 - 15 tons of weight into a 20' Container.
But why don't you ask the Rice Supplier to tell you?
It will also depend if they are on pallets or packed loose.
You can pack 20mt weight and 32cbm into a 20-feet. Above 16mt however you will pay an overload surcharge. I don't know the volume of the rice sack, but according to weight up to 400 sacks would fit into the container.