Koha'sKhoa's second option is the preferred one. Anyways, this sometimes does not work for bundles. There you have to also carefully check the bundles option's and quantities.
The logic for the default quantity could be found in
\Magento\Bundle\Block\Catalog\Product\View\Type\Bundle\Option::getDefaultValues
In my case a required option did not have a default option, leading to a zero default quantity.