Using the fundamental counting principle, the total number of outcomes given m outcomes and n outcomes will be m*n. A helpful way to think about this is by using a tree.
Say we have 2 shirts and 3 pairs of pants. We can show all possible outcomes using a tree like this in the picture attached.
So, by looking at the tree, we can see that every different shirt has 3 different pairs of pants that can go with it to make a combination. Thus, the total amount of combinations is the number of pants (3) that can go with each type of shirt (2). So, 3*2 is 6 total combinations.
In this example, m was 2 and n was 3. Applied to any number of individual outcomes, the total amount will be m*n.