You can follow this instruction for How to show Quantity Selector & Add to Cart Button in Same Line For Shopify Dawn Theme and other free 2.0 Shopify's themes. Please follow these steps.
- Login to your Shopify store
- Online store > Themes > Edit code
- Open base.css file
and paste these code below
.button--full-width {
width: calc(100% - 150px) !important;
margin-top: -72px;
margin-left: 150px;
}