Avogadro's Number, NA, corresponds to the number of items in a mole. It is formally defined as the number of Carbon-12 atoms in 0.012 kg of Carbon-12.
At present it is not technologically feasible to count the exact number of atoms in 0.012 kg of Carbon-12, so the precise value of Avogadro's Number is unknown. The 1998 CODATA Recommended Value for Avogadro's Number is 6.02214199(47)*1023 . Where the number in parenthesis represents the one-standard-deviation uncertainty in the last digits of the value. This number can also be expressed as:
A number of methods can be used to obtain estimates of Avogadro's number. One modern method is to calculate Avogadro's number from the density of a crystal, the relative atomic mass, and the unit cell length determined from x-ray crystallography. Very accurate values of these quantities for silicon have been measured at the National Institute for Standards and Technology (NIST) and used to determine Avogadro's number.
1. Journal of Physical and Chemical Reference Data, 28 (1999) 1713.