The possible accuracy is limited by uncertainty over mass of IPK(International Prototype Kilogram), which is 20 ppb. You cannot measure the number of atoms in a kilogram of silicon better than that, because a kilogram is <i>defined</i> by IPK.<p>20 ppb translates to uncertainty of about 10000 trillion atoms. The number will be set to a round number, because any number within trillions is compatible with the legacy standard.