Answer:
5 cubes
Step-by-step explanation:
The figure consists of three layers.
The top layer consists of one cube (this cube is visible).
The second layer consists of 3 cubes (two of them are visible and one is not visible - placed under the cube of the top layer).
The third layer (base layer) consists of 5 cubes:
So, 5 cubes are needed to build the base of the figure.