Hello,
Yes there are 20 levels according to Unity Editor, but four of them are main menu, map, cut-scene, more levels screen and the rest 16 are actual levels.
I already have plans to add them in future updates that's why their achievements are available. But for now they don't affect unlocking 'Naked' skin. Completing any 15 achievements can unlock the skin.
Head hunter are not like Xenomorphs. Those aliens will have their own game. As the name suggest they will be only interested in getting oral.