I have an old "Courses of Study" book, I'm sure you have a more recent one. Under the Arts & Sciences section about Registration and course Scheduling there is a paragraph about "Maximum number of credits per semester". It says "First-term freshmen must petition to register for more than 18 credits, other students may register for up to 22 credits if their previous term's average was 3.0 or higher and they are in good academic standing. No more than 22 credits may be taken in a regular semester without permission of the college faculty's Committee on Academic Records. Students who fail to receive approval for the excess credits from the committee may count only 18 credits for the semester toward the degree".
I think any more than 20 is ludicrous. Personally, I wouldn't take more than 17 if I didn't have to. I aim for 14-17 each semester, which is enough to keep me busy, but not so much as to stress me out.