How to determine the numbe of particles in a given direction

Dear Sir/Madam,

I am recently looking through the Heat transfer API code and a question comes to my mind.

In the API code, we usually specify element1 and element2 and then perform the subsequent operations.

However, I was just wondering if there is any function to determine the number of particles in a certain direction. 

For example, in the following figure, there are 4 columns of particles, each with 3 particles. What function can be used to determine that the number of particles along the y-direction is 3?

Kind regards,

Chao