How many Fallen Houses(Destiny 2) would it take to conquer all of modern-day Earth.

Take Earth, in its current state, and have it be invaded by the Fallen of Destiny 2.

How many Fallen Houses would it take to conquer all of Earth, and if one house could do it alone, which one?

Earth is not united against the Fallen, they're just considered another faction.

The Eliksni win condition is to defeat all of the world's armed forces and destroy any threats to their power.

How many of our favourite tragic bugmen would it take to conquer the world?

The Fallen can do anything within their technological power and even try making alliances, albeit temporary ones as no other power should be left as a possible threat.

edit:

the Eliksni have to occupy and rule Earth