Avogadro’s number (Avogadro’s constant) is the quantity of objects found in 1 mole of a sample. It is an exact number that is defined as 6.02214076×1023 mol−1. This is essentially the number of atoms in 12 grams of carbon-12.
When you perform calculations using significant figures, you use the smallest number of significant digits. So, if you are using a mass of carbon that is 12.01 grams/mole (4 significant figures), you use 6.022 x 1023 as Avogadro’s number (4 significant figures).