what is the value of (delta) ng for the following reaction :-

H₂(g) + I₂(g) -- 2HI(g)

$∆$n_g stands for the change in the number of mole of gaseous species, when a reactant combines to form products. 
Mathematically it is given by the formula 
$∆$n_g = (n_p-n_r)g
​Where, np stands for the sum of the number of moles present in gaseous species and nr stands for the sum of the number of moles of gaseous species present on reactant side. 
For the mentioned reaction, i.e. H₂(g) + I₂(g) -- 2HI(g)
As all the compounds are present in gaseous form therefore 
n_p = 2 and n_r = 1+1 
= 2
Hence 
$∆$n_g = (n_p-n_r)_g​ = 2-2
= 0