Is it possible to POST more than one article at once?
As far as I know there is now way to create multiple nodes in one request. You can send multiple requests instead. 

The JSON format need not to to be that much complex(as you mentioned in your POST).

You can use the following JSON format to create a node.


  URL : http://www.example.com/d8/entity/node
  
  Method: POST
  
  Headers: 
  
  Name: X-CSRF-Token
  
  Value: TPlhcaO_KESksKPye805T86j0-ja9rhyfaaqi6zpA8


You can get user session token by requesting the following URL

URL: http://www.exmaple.com/rest/session/token

JSON format to create a node  (Article) for D8

{
  "type":[{"target_id":"article"}],
  "title":[{"value":"Hi World"}],
}


I've tested this on my D8 instance. It's working fine. Hope it helps