What prefix indicates a quantity of seven?
A couple of examples:
septagon - a 7-sided polygon
septuagenarian - someone who is between 70 and 79 years of age
September used to be a good example, up until the Roman Emperor Julius Caesar (who reformed the Roman calendar) and Augustus (who also wanted a month named after himself). Before Julius' reforms, we had (I've added the hyphens):
After the months were added for the emperors (July for Julius and August for Augustus), September became the 9th month and so on down to December being the 12th month.
Here's a little video on the reformation of the Roman calendar and Julius Caesar: