Hi,
You can change that option to "show select by price" and enter prices in your product for different minimum quantities and the system will pick all these minimum quantity of the prices as values in the dropdown.
For the "show select", we limit to 15 as to not have an enormous dropdown. If you set a maximum quantity per order in your product, the system will automatically use that value to know the number of values you want in the dropdown.
If you want to change the way the dropdown is generated to add more values, turn on the "display view files" setting of the HikaShop configuration in order to know how which view file is used to generate the dropdown on your website (it can come from different files based on how you configured your HikaShop and which page you want to do it). Once you know, edit the view file via the menu Display>Views and you'll be able to change the code for that.
For example, if it's the view file layouts / quantity.php , you'll find that line:
$max_quantity = (int)$min_quantity * 15;
where you can change the 15 text to what you want.