What does "DWELLERS" mean?

When I look into upgrading certain rooms, there is a table that shows "Dwellers", then states a CURRENT number and an UPGRADED number.

What are those numbers counting? The amount of dwellers that can work in that room?

Solution 1:

Dwellers in this instance is the number of vault dwellers that room increments your maximum vault dweller capacity by. For example if you have a maximum dwellers capacity of 10 and upgrading a room will add another 4, after upgrading the room you will have a maximum dweller capacity of 14.

Dwellers will only be stated on living quarters as the other rooms provide alternate resources (such as water or power) and will thus show these statistics when you open the room upgrade screen.