A sample of a given element can consist of a single atom of that element, and so there is no fixed quantity of molecules required to make up an element. In the universe at large, however, hydrogen is by far the most abundant element.
Universe Today reports that the observable universe could contain between 1 quinvigintillion (1 with 78 zeros behind it) and 10 sexvigintillion atoms (1 with 82 zeros behind it). The vast majority of those atoms are hydrogen that formed during the early history of the universe.
Heavier elements are less abundant. Oxygen is the third-heaviest and third-most-abundant element, followed by carbon. "Dioxide" is not an element but a chemical name for molecules with two oxygen atoms.