If the number is small, you can use multiple detail sections (Detal a, detail b, ...) and suppress each section based on an escalating comparison to the quantity field.
If you need to handle large quantities, see an FAQ I wrote a bout this at: Duplicating Records based on a Quantity Field - Business Objects: Crystal Reports 1 Formulas FAQ - Tek-Tips
Alternatively, there's at least one 3rd-party Crystal Reports desktop scheduler (see list of such tools at http://kenhamady.com/bookmarks.html) that can automate the process of printing copies based on field/formula and setting dynamic expressions (such as Copy N of M) in the printouts.