[2019] Killed 9 hunters and got the masks but only 5 keys have dropped.

Correct, the various Hunters need to be killed in specific ways by following their clues and solving their puzzles in order to drop the keys. Or they can be killed any such way and drop a mask. Once you kill a Hunter in your game, you will have to join someone else and help them kill the Hunter to have another shot at the key. So it's best to be sure of the puzzle before eliminating the Hunter.

Measure twice; cut once and all that.