Actually issue in your theme product.tpl file. You can comment this code from your tpl.
or remove with take a backup of file. Then it will be work fine. In your product.tpl file, there are two quantity_wanted boxes. One is hide by css and another one is showing on proper position. So please remove my quoted code.
You can check this screenshot https://s3.amazonaws.com/diigo/9027923_309703947_22549225?AWSAccessKeyId=0R7FMW7AXRVCYMAPTPR2&Expires=1459693039&Signature=BVbY25CmDklfrxbrWudn%2FvI5tPw%3D