How to build 643 rooms in Fallout shelter?
You cannot get 643 rooms in fallout shelter, I don't know where you got this number. According to the wiki the actual maximum amount is lower, much lower
Source
There are 25 floors; all floors can hold eight rooms. In addition all floors can hold up to two elevators. Adding more than two elevators per floor will sacrifice space for other rooms.
so 25*8=200 rooms