Energy band diagrams in metals, semiconductors and insulators
In a metal the conduction and valence bands overlap so electrons can easily pass into the conduction band. This means that the number density of free charge carriers will be very high.
In semiconductors there is a gap between the valence band and conduction band, also the valence band is completely full. If some electrons in the valence band are provided with the right level of energy they can jump the gap into the conduction band. This energy can be provided by thermal energy. However the number of free electrons in the conduction band is still much less than that of a conductor.
Note some conduction can take place in the valence band provided it is not completely full. For every electron that moves into the conduction band it creates the equivalent of one mobile charge carrier in the valence band.
In an insulator the energy gap between the valence band and the conduction band is much larger and a large amount of energy would be needed for electrons to jump into the conduction band. For this reason there are far fewer electrons in the conduction band of an insulator than that of a semiconductor.