Matteo,
You may want to use one WTPart represent the final solution that will carry the ID Number that will be used. To capture different design alternatives you could give the CAD files a temprary Number and use rename later when a single alternative is selected. (If the rename Action is restricted, then you could do a Save As of the selected final design model). I would restrict the use of the "temporary ID's" to internal use. When communicating externally the need for unique and definitive identification may outweigh the "cost" of using up Drawing Numbers.