How many molecules are present?

Question: How many molecules are present in 4.21 mol of hydrogen gas?

Answer: 1 mole of anything contains 6.02 x 10²³ particles.
(Note that "mol" is the abbreviation for mole.)
So, 4.21 mol of hydrogen gas contains 4.21 x 6.02 x 10²³ = 2.53 x 10²⁴ molecules of hydrogen.

You can find a tutorial with worked examples at http://www.ausetute.com.au/namole.html