The local bottling plant of a soft drink manufacturer fills $5,000$ bottles every $2$ hours. The plant purchases the plastic bottles from a small, nearby supplier that requires a lead time of $30$ minutes for a container of $250$ bottles. The bottling plant keeps a safety stock of $10\%$ of lead$-$time demand and is considering switching to a kanban system. How many kanban containers $($or cards$)$ should the bottling plant utilize to manage the replenishment of empty bottles?