There is no strict ruling that an incomplete or demo submission counts against any submitting teams, however the trend has been more often than not completed but short VNs have been leaders in terms of judging over the past two years.
As long as what you worked on is submitted before the deadline, it will be counted whether it's a complete experience or not. Many teams make demos as part of this jam to begin work on a product they intend on finishing at a later date, and we love that.