An issue number is included in every newsletter on the cover page. iNewsletter allows you to use automatic issue numbering ("Issue 1", "Issue 2", "Issue 3", for example), or you can override it in each newsletter ("Special Edition", "Term 1, Issue 1", "Term 1, Week 1", for example). If you override the numbering in a newsletter, the next newsletter will resume the automatic numbering (unless overridden again).
On the Dashboard click the Settings > General link in the left bar
Under the Next Issue Number section, enter the next issue number you want your next published issue to appear as.
Click the grey Save button
All future newsletters in this publication will automatically increment from this issue number, unless overridden in a newsletter.