Avogadro’s number (Avogadro’s constant) is the quantity of objects found in 1 mole of a sample. It is the number of atoms in exactly 12 grams of carbon-12. The value is determined experimentally, so there is error in Avogadro’s number, as with any measurement.
So, while we have a good idea of what Avogadro’s number is, it’s not an exact number. You can express it as 6.02214179(30)×1023. However, for most chemistry calculations, you will be working with atomic masses, which are commonly reported using 4 significant figures.
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 would use 6.022 x 1023 as Avogadro’s number (4 significant figures).