In naming a binary molecular compound, the number of atoms of each element present in the molecule is indicated by what?
You use prefixes (from Greek).
The prefixes meaning:
mono=1, di=2, tri=3, tetra=4, penta=6, hexa=6, hepta=7.
Mono is often left out, unless there are similar compounds with di, tri, or otherwise. Also mono-oxide is often shortened to monoxide.
Water would be called di-hydrogen mono-oxide.