The number of entities (e.g. atoms, molecules, ions, particles etc.) in one mole of a substance is given by Avogadro's number :
$N_{A}$ represents Avogadro's number.
$N_{A}$ = 6.023 x $10^{23}$
Example 1
How many atoms of C are contained in 1 mole of $CO_{2}$?
From the chemical formula, we see that 1 mole of $CO_{2}$ contains one mole of C atoms and 2 moles of O atoms.
1 mole of C atoms = 6.023 x $10^{23}$ atoms of C.
Therefore, there are
6.023 x $10^{23}$ atoms of C

in 1 mole of $CO_{2}$.
Example 2
How many molecules of $CO_{2}$ are contained in 4.40g of $CO_{2}$?
Steps:
 Convert mass to molecules
 Express molecules in moles
Step 1:
Mass of C is 12g
Mass of O is 16g
Therefore, molecular mass of $CO_{2}$ is 12 + (2 x 16) = 44g
1 molecule of $CO_{2}$ contains 44g
so, 4.40g of $CO_{2}$ will contain
$$\dfrac {4.40}{44}=0.1$$

Therefore, 4.40g of $CO_{2}$ will contain 0.1 molecule.
Step 2
Since each molecule of $CO_{2}$ contains 6.023 x $10^{23}$
number of molecules in 4.40g of $CO_{2}$ is
0.1 x 6.023 x $10^{23}$ = 0.6023 x $10^{23}$

or 6.023 x $10^{22}$