Assuming you are creating a jpeg file for each slide scanned, then each file will have a date/time stamp on it. If you open the directory containing the files you can sort them into time order ie, the slide you scanned first will be at the top of the list. You can now click on the top file, CTRL+A to highlight all the files, right-click, choose rename and type eg file (1). The first file will be renamed file (1), the second file (2) etc. So now you have the files in the order in which you scanned them with consecutive names.
Is that what you are trying to do?