The UI view of the dataTable reflects the contents of that dataTable's corresponding DataModel object. Therefore to display a table with a fixed number of empty rows, you must create a DataModel that contains that number of empty row model objects.
If you wish to edit/save rows, do not take the common shortcut of using a Collection or array as a DataModel. Use an actual javax.faces.DataModel class object such as a ListDataModel or ArrayDataModel. The DataModel classes decorate the underlying data with extra location information that permits the action logic to know which row the user is acting on.
An IDE is no substitute for an Intelligent Developer.