I'm leaning congruency equations, so for example:
$$
 ax \equiv b \pmod m
$$
I have that the number of solutions will be equal to $d$, where
$$
 d = \gcd(a, m).
$$
And the solutions ae:
$$
 x, x+m/d, x+2m/d, x+3m/d, \ldots , x+(d-1)m/d
$$
Now, having understood how I solve the equation, I am still not entirely sure as to why the number of solutions is given by $d$...

Anybody can calrify please?

Cheers! :)