See this: http://blog.wenxuecity.com/blogview.php?date=200708&postID=19650
The visa # will be assigned until the VO consider your case is approvable. But those contractors who collect information for the VO need to check whether your visa is available before they give it to the VO. So, if your case is in the VO's hand, that means you should be able to have one visa #.