Crystal Structures of Metals and the number of atoms in metal object

In the BCC (Body Centered Cubic) structure each unit crystal cell will have 1 atom in the centre and 8 on the unit cell corners. So practically only 2 atoms will be in one unit cell. Remember each corner atom will be shared by eight unit cells (to understand it make a diagram).

On the other hand in FCC (Face Centered Cubic) structure there will be 6 atoms in the centre of the six faces of the unit cube while 8 atoms on each of the the corners. The corner atom will be shared by eight unit cells while the face centre atom will be shared between only two unit cells (again make a diagram to understand). So practically a unit cell will have 4 atoms to its share. It clear shows that in FCC structure more atoms are packed than the BCC structure. That explains for the more density in case of FCC structures. The packing ratio of FCC is 0.74 as compared to 0.68 in case of BCC.

I agree fully with the answer explained by Neeru Bhatt as above and now try to explain your further query in the matter.

Crystal structures are explained with these geometrical placements of the atoms. You are getting confused because you are taking a particular material and placing its atom in these two designs. Do not put same material in two designs. It is not possible logically.

The fact is that the natural arrangement of atoms of a material can be in a particular crystal structure and that gives it its density, appearance and overall physical properties. You can not have a change of structure for it. Yes, another material can have same or different structure but it will not look like the first material. You are getting confused because you are putting same material in different structures. It is a natural binding design and it is not in our hands to change it. We are only trying to understand it to calculate packing ratio and other aspects.

Please also note that the basic unit crystal cube will be of different size for different materials. I hope that clears the confusions in your mind.

Again, thanks for the answer.
I may be confuse and my academic qualification is as architect but still…
First I don't understand what do you mean that I put the same material in to two designs. However can you please just answer me once more?
Is it right to say that if piece of material (steel) comprising of 1000 atoms and its size is 10x10x10mm (figures just for the discussion) and it's at its BCC phase, have the same size and the same atoms count at its FCC phase because the FCC unit cell lattice parameter (0.359nm) is to some degrees bigger than the BCC unit cell lattice parameter (0.287nm)?
I am sorry if I may sound lame, but as I mention I am architect / blade smith.