The number of atoms needed such that the number of grams of a substance equals the atomic mass of the substance,

An Avogadro's number of substance is called a mole. For example, a mole of carbon-12 atoms is 12 grams, a mole of hydrogen atoms is 1 gram, a mole of hydrogen molecules is 2 grams, etc.

Avogadro's Hypothesis, Einstein, Mole, Universal Gas Constant