A shame the dark alliance didnt become a temporary ally...
But yeah an empire encompasses a very large territory and can include many traditional kingdoms and peoples but is controlled by one sovereign ruler often called an emperor, whereas a kingdom is any size of land ruled by an actual king. It is a matter of definition but there seem to be under 200 empires documented in total throughout all of recorded human history putting it into perspective. That is a small number in case it was not clear.