The standard is 16 credits. If you average 16 credits per semester for four years you will get the number of credits total needed to graduate (128). However you can range from 12-20 any semester. I find 18 credits or more will be hard to handle, especially if you have any labs, recitations, etc.
Don't worry about having 32 credits lined up. When I was a freshman I wanted to take like eight classes, and I asked the same question. Pick out around 16 credits (four classes) and plan on taking the others second semester. You can't enroll for second semester yet though, not until November I believe.