how do you know when you have found all the multiplication combinations for a cetain number?

Answer 1

Do you mean like:
3 * 1, 3 * 2, 3 * 3, 3 * 4 -- and so on and so on until you get to 3 * 1,000,000,000,000?

Answer 2

i think Gina may mean the factors of a number, like the factors of 12 are 1,2,3,4,6,12

one way is to start at the lowest factor and write down its pair, ie: 1*12, then 2*6 then 3*4, when you get to 4*3 you have started to overlap meaning that you have reached the maximum number of combinations.