If you are adding new inventory items, you can't display the entry id because the entry ID doesn't exist until the form is submitted. If by "fill in data for inventory" you mean they are editing an existing inventory entry, then use the [id] shortcode as the default value for a read only text field or as a text string you display on the form. Don't allow the entry ID to be editable. It will break the connection between the form and its fields.