Look at it this way.
Assume each of your credits requires 3 hours of studying a week.
So that means, you take 12 credits, + 36 hours of studying. That's 48 hours a week of work.
Now let's take 15 credits, + 45 hours of studying. That's 60 hours of work. You're working half a day more by taking 3 extra credits.
With 18, 54 hours of studying. That's a whopping 72 hours of work a week.
A week has 168 hours.
Let's say you sleep 8 hours a day (hah, as if). That's 56 hours, so in reality, we have 112 hours to work with. Assume travel time to and from classes, and doing regular stuff like dressing, bathroom, etc. That's an half an hour a day for guys. 4 hours rounded. We now have 108 hours. We have to eat, too, so that's about 2 hours a day if you have long meals. 14 away. 94 hours. So that means that if you're taking 12 credits, you have about 50 free hours. 15, you have about 38. 18, you have about 24. This is an example, not a schedule, but the more free time you have, the less pressing your schoolwork is, and it also gives less information about how well you balance your time. If you can pull 18 credits and get good grades, that may look better than a person with 12 credits getting all As, as you're putting more time out of your week to work, and you have less spare time to work with if you need to study something more, for example.