OKay so i'm kind of stupid and I don't understand where the 2 comes from. How do you get a 2? If nickel is a transitional metal and has 2 different charges of 2 or 3 how do you know that it's a 2 in NiCl2 if Chlorine has a charge of 1? Shouldn't it be NiCl ? I'm really confused and i hope that someone can explain this to me. Hopefully that this has made some sense.

Chlorine is in group 7 and can only take 1 electron to make it`s 8 filling that outer shell.

if Ni has to 2 electrons to give away, it`ll need to Chlorines two accept them, one each.

I think the problem is how do we know it's not, for example, NiCl3.

That's not a trivial question so I don't think you are being stupid. The old answer was to make lots of compounds like chlorides, sulphates, oxides and whatever then analyse them to see how much of each component there was in each compound then try to work out how they could agree with each-other and with things like how much they depressed the freezing point of water when you made solutions of them and which ones crystalised in the same shapes.

The current way is just to learn the valencies of the elements.

This "if Ni has to 2 electrons to give away, it`ll need to Chlorines two accept them, one each." is a question rather than an answer. Why 2? why not 1 or 3?

The answer is to do with things like lattice energy and ionisation potential.

I believe that you are confusing charge with number of atoms and superscript with subscript:

NiCl2 is One Nickel with Two Chlorines (the 2 should be subscript in this case)

One divalently charged Ni (++) is bound to two Cl(-): Ni++(Cl-)2

Charged nickel alone could be written Ni(superscript2+ or superscript++): Ni++

Charged Chlorine could be written Cl(superscript1 or superscript -):Cl-

Does that help or confuse you more?