Actually Robert you shouldn't be getting sequential numbers like that. If
you image on multiple nights it should just resume numbering. I'm guessing
that somehow you are resetting your sequence so it starts over at 1 instead
of 5 (or whatever).
However, you can adjust the progress of each event. On the event, to the
far right under the progress field, just double click on the number (like
0/10) and you can manually adjust the number to show how for the progress
has completed. So if after the first night you have 4 images, just set
that number to 4/10 which tells SGP that 4 images out of 10 have been
completed and it will start the new numbering at 5.