What is Avogadro number? Write its value.

(N/A) The number of molecules contained in $1 \text{ mole}$ of a substance is called the Avogadro number. It is represented by the symbol $N_{A}$.
$N_{A} = 6.022 \times 10^{23} \text{ mol}^{-1}$.
Modern definition: The number of atoms contained in exactly $12 \text{ g}$ of carbon-$12$ is called the Avogadro number.
Avogadro number is a universal constant applicable to all types of fundamental particles:
$1 \text{ mole of atoms} = 6.022 \times 10^{23} \text{ atoms/mole}$
$1 \text{ mole of ions} = 6.022 \times 10^{23} \text{ ions/mole}$
$1 \text{ mole of electrons} = 6.022 \times 10^{23} \text{ electrons/mole}$