Question

how many atoms are there in one molecule

Answer 1

depends on the valency of the element and how many atoms are required by the element to form one stable molecule of the element

Answer 2

A mole is a convenient counting unit whenever one is dealing with numbers of atoms or molecules. It is equal to Avogadro's number (NA), namely 6.022x1023. If we have one mole of water, then we know that it will have a mass of 2 grams (for 2 moles of H atoms) + 16 grams (for one mole O atom) = 18 grams.