To determine factor pairs you can make an iterative loop, and check for how many times your input value is evenly divided by a number, ie. for 6 you would check:

6 % 1 = 0, 6 % 2 = 0, 6 % 3 = 0, 6 % 4 = 2, 6 % 5 = 1, 6 % 6 = 0

There are 4 zeros, so 6 has 4 factors.

Determining factor pairs takes a little bit more work than you would imagine, as you cannot simply divide by 2, because of square numbers, who have an odd number of factors.

Instead, you can ignore 1 as a factor, divide the amount of other factors by 2, then add 1 to your amount of factor pairs.

This process may look like:

int i;
for (i=2; i<=input; i++) {
    if ((input % i) == 0) {
    factorpairs++;
    }
}
factorpairs = (factorpairs / 2) + 1