The no. of atoms in 1 mole hydrogen is twice the no.of atoms in one mole helium. Why ?

Avogadro's Law states "Equal volumes of gases, at the same temperature and pressure, contain the same number of particles, or molecules."

A mole is 6.022 * 10^23 particles (atoms or molecules, dependent upon the gas).

Hydrogen gas is diatomic (H₂). Helium gas is monatomic (He). Hydrogen will therefore have twice the number of atoms.