How does the modulus operator function

Question

Assume I need to prepare an array's output to display a specific amount of entries per line. How do I achieve it using the modulus operation?

The code below in C++ displays 6 items per line, but I have no idea how or why it works.

for ( count = 0 ; count < size ; count++)
{
    cout << somearray[count];
    if( count % 6 == 5) cout << endl;
}

What if I want to show 5 items per line? 

How can I locate the correct expression?