A mole of any material contains 6.023 x 10^23 atoms. The number 6.023 x 10^23 is also known as the Avogadro's number. Thus 1 mole of an element will contain an Avogadro's number (or 6.023 x 10^23) of atoms.
Elements are characterized by atomic weight (sum of their protons and neutrons). For example, sodium has an atomic weight of 23 (11 protons and 12 neutrons). This means that 23 grams of sodium constitutes 1 mole of it. In other words, 23 grams sodium contains 6.023 x 10^23 atoms. Thus, given a certain quantity of sodium, we can determine its mass and calculate the number of moles and hence the number of atoms in it. Say 46 grams of sodium is given to us (something that can be measured by a simple measuring scale in lab), this means 46/23 = 2 moles of sodium. In other words, 12.046 x 10^23 atoms (2 x Avogadro's number) are there in the given mass of sodium.
Hope this helps.
one mole =6.023 x 10^23 atoms
n moles=n*6.023 x 10^23 atoms