Thank you so much. I think I get it now, and here is my understanding:
If we assume two species (x,y)
$$\Sigma_{mix}= \frac{No\ of\ X\ atoms}{Volume\ of\ X} \times \frac{Volume\ of\ X}{Volume\ of\ X+Y} + \frac{No\ of\ Y\ atoms}{Volume\ of\ Y} \times \frac{Volume\ of\ Y}{Volume\ of\ X+Y}$$