It is known that an ender dragon is counted as 9 different entities when using

/kill @e[type=ender_dragon]

However, attempting to kill even one of these "entities" causes the entire dragon to be killed This can be tested by using the following command:

/kill @e[type=ender_dragon,limit=1]

Finally, trying to modify the kill command further, by using a different sorting method, will result in the dragon having nine lives, requiring you to run the command 9 times

/kill @e[type=ender_dragon,sort=nearest,limit=1]