Question

How valency is calculated

Answer 1

by cross criss method:
eg: valency of carbon in CO2
so, valency of carbon is 2 and of oxygen is 1.

Answer 2

See, if we wanna know valency of an element.
it is the number of electron transferred (accept or donate) to complete its inertness.
For ex: Na (2,8,1) having 1 valence electron and it can donate that electron to to complete its inertness(2,8). So for Na, 1 is the valency.
Now, if we talk about N(2,5) having 5 valence electron and it can't donate 5 electron but it can accept 3 electron to complete it's inertness(2,8)

So, For metals, valency = valence electron
For non metal, valency = 8 - valence electron

Now, if we wanna know valency of element of a compound, it is actually done by drawing the structure(for accuracy).
eg, C in CO₂

⁻²O=C⁽⁺²⁺²⁾=O⁻²

Every compound exist when they are stable, both Oxygen having -2 as valency, so to balance its charge, C has the valency 4 i.e  2+2 
One more example, 
Hydrogen peroxide H₂O₂

If we calculate mathematically, we might be get wrong, so let's see diagram.

⁽⁺¹⁾H-⁽⁻¹⁾O-⁽⁻¹⁾O-H⁽⁺¹⁾

Here, the valency of O is --1 and not -2. We can see the whole compound is electrically neutral (+1-1-1+1) =0

General method to find valency,
eg. Mn in KMnO₄
Let is be x
we know, K has valency 1
O = -2
so, 1 +x +4(₋2) = 0
⇒x = 7

I hope, this will help