soprano parts

Are Mezzo Sopranos Rare? They have a range from approximately the G below middle C (G3, 196 Hz) to the B two octaves above middle C (B5, 988 Hz). Some coloratura mezzo-sopranos can sing up to high C (C6, 1047 Hz) or high D (D6, 1175 Hz), but this is very rare. Is mezzo-soprano common?