Several.
Several.
The use of the word "cubes" in the question is kinda throwing me off. A cube by definition is symmetrical and comprised of six equal squares and it's hard to see how the given pieces smoothly form "identical cubes" to be fused into some larger "object," whatever shape that might take.
Whoops -- I had jumped directly from the subject line to the image, so I missed the six cubes.
These are side/top/bottom views of an object constructed by joining six identical cubes together. Also, I suspect that the five given views are not necessarily oriented in any way.
I think that a rectangle signifies two adjacent cubes located the same distance from the viewer. In other words, the faces of these two cubes form a rectangle because each face lies in the same plane. (There could be another cube behind the rectangle, in a view.) But a square next to a rectangle indicates that the face we see as the square lies at a different distance (closer or farther) than the cubes forming the rectangle.
Looking at #5, there are three cubes whose faces are at the same distance, and there are two other cubes whose faces are at a different distance. Where is the sixth cube? I think I can imagine its position, to render the other given viewpoints.
If I'm correct, the missing view will look like an L-shape, with a square attached to one of its longer sides. :cool: