How many atoms make an atom?

1 mol of He contains apx 6.02214x10^23 atoms

It will vary from person to person, but essentially the technology does not exist at this time to determine an exact number.

That varies depending on the element and the isotope. An element is defined by its number of protons. This is called the "atomic number" of an element.

2 hydrogen atoms and one oxygen atom produces water molecules

Inadequate question; carbon cannot derived from helium in laboratory experiments. But carbon is formed from helium in giant stars following a very complicate system of nuclear reactions.