There are several rooms in the game which drop stacks that need to be picked up; the Mine, the Farm and the Vault, for instance. Each of these have pieces of furniture that increase the stack size, but take up one of the tiles a stack can form on. So as you keep adding stack expanders, the stack size (and thus the maximum capacity of the room) grows, but there will be fewer and fewer possible stacks. At one point there will be so little space the maximum capacity starts shrinking again.
The following table lists, for the given number of tiles available in the room and the given furniture bonus, the optimal amount of stack expanders. Mind that the number of tiles only counts those tiles upon which furniture can be placed (or stacks can form).
For rooms with other numbers of tiles, you can use the following formula to determine the optimal amount of stack expanders:
where s is the number of free tiles, and b is the bonus from the stack expander (0.25, 0.5, 1, 2, or 4).
The greatest integer value for n will be the optimal amount of stack expanders.
Easy way to count optimal amount:
|+25%||+50%||+100% or better|
|Even amount of free tiles||Tiles / 2 - 2||Tiles / 2 - 1||Tiles / 2|
|Odd amount of free tiles||(Tiles + 1) / 2 - 2||(Tiles + 1) / 2 - 1||(Tiles - 1) / 2|