Hello. The extension Minimum quantity of configurable product will be the perfect fit for what you have asked. It applies minimum quantity for configurable product as the compulsory condition for customer to proceed to checkout. Admin can flexibly set minimum quantity for a configurable product, specific configurable products or all configurable products site-wide.
If you wish to know more information, please refer here http://bsscommerce.com/magento-minimum-quantity-of-configurable-product.html