The daily demand for a product is $8$ units. Since the plant operates $300$ days per year, this leads to an annual demand of $2,400$ units. The setup cost is $$40$ per setup and the carrying cost is $$80$ per unit per year. The daily production of this product is $12$ units. Lead time is four days, and safety stock is $5$ day's production. What is the optimal number of kanbans? $($Answer to two decimal places, e$.$g$.,10\mathrm{.}01)$