In chemistry, how do you know what number to put after an element? e.g. there's a 2 after oxygen.?

While many elements exist as single atoms, there are ten that are diatomic - H, O, F, N, Cl, Br, and I. So these would also have the subscript 2 after the element, when it is in elemental form. Also, phosphorus exists as P4 and sulfur as S8. Actually, after rereading your question, it occurred to me that you might be talking not about subscripts, representing the number of atoms, but rather the oxidation state of that atom, which can vary. So a number like -2 after an O tells you that the O atom has acquired 2 additional electrons and is now an oxide anion. Nitrogen can have oxidation number ranging from -3 to +5, depending on the chemical compound.

It's how many atoms of that element are in that molecule. For example, there are two hydrogen atoms in one molecule of water. H2O You could be talking about the way we write metal ions too. For example, Iron (II) has a charge of 2. Oxygen isn't a metal though, so this probably isn't what you wanted.

it represents the number of that particular element in the compound/molecule/ formula

It's how many atoms of that element are in that molecule. For example, there are two hydrogen atoms in one molecule of water. H2O You could be talking about the way we write metal ions too. For example, Iron (II) has a charge of 2. Oxygen isn't a metal though, so this probably isn't what you wanted.

it represents the number of that particular element in the compound/molecule/ formula