I usually take 19 or 20. I had 22 first term, but that was because of an extra little project I did before term started for a bonus
I had 19 last semester, and honestly, I didn't think it was that bad. Honestly, I didn't really notice a huge difference in workload from first semester, where I had 15 credit hours. I did, however, feel like I wouldn't have enough free time to comfortably take another class on. That could have been because I was in a 2 credit hour class where we were usually in class for 5 hours a week, though (plus we were expected to spend a lot of time outside of class working on our projects with our team. probably an average of 4 hours or something like that), so that could have amounted to the allotted time of a 4-5 credit hour class.
What classes are you taking if you have 10 classes and 20 credit hours? An average of 2 credits per class doesn't seem right if you're taking engineering, physics and mathematics courses. The ones at my university are normally 4+ credits. Last semester (19 credit hours), I had 5 classes.
Yeah, a requirement of 16 credit hours (to graduate in time) isn't that bad. It's probably slightly less for a computer science major, but I have minors in mathematics and physics, so it probably chalks up to around 16 as well.