When you save the draft the program will assign a draft number. The same thing would happen for the invoices. Now, if you click on preview for either a draft or an actual invoice you would create then the preview won't show the number because the draft or Invoice has not been saved.
If you have already saved drafts or created invoices then you may go to View > Invoices and click to highlight the invoice you want to check, and then click on preview. You may do the same thing but select the invoices saved as Draft so you can preview them as well. For more details you may access the user's guide by clicking on the help icon located at the upper right hand corner of the program's window (Questions Mark Icon for Windows and Lifesaver for Mac).
