Sold Individually
In the Inventory tab, there is an option to control the universal quantity box for smart bundles, which is “Sold Individually”.
When this option is enabled, the universal quantity box next to the Add to Cart button will be hidden. Therefore, one buyer/customer can only add 1 unit of this bundle to the cart and place an order. This option is to make sure each customer can only purchase 1 unit of the chosen bundle per single order.
If you want to allow multiple bundles to be purchased in a single order per customer, remember to uncheck/disable this option from the Inventory tab of the bundle page.