Why is the number of electrons per unit volume , n fixed in a conductor regardless of the temperature while its not in semiconductors ??
I think you mean number of free/conduction electrons.
In a semiconductor, electrons can promoted to the conduction band by thermal energy; i.e. by raising the temperature of the material. In a conductor, the number of free/conduction electrons is more or less at saturation, and heating the material does not produce any more.
Of course the total number of electrons in the material is not affected by temperature.