This past semester I did physics II (4 cr), physics lab II (1 cr), biology II (3 cr), biology lab II (1 cr), calculus II (4 cr), chemistry independent study (1 cr), and English II (3 cr) and was working about 20-24 hours a week. I thought it was pretty manageable, I got an A- in physics lab II as my lowest grade [hopefully, I haven't gotten my calc grade back yet].
I think it depends on what kind of work you're doing. I got a lot of study time while I was working, which is a big part of why it was so manageable for me. Also factor in whether you've taken any of those courses in high school. Calc and physics were new to me this year, so I had to study a little more than I probably would have had I taken the high school version. If you want to take more, maybe go with a couple easy classes if you don't feel super comfortable with the material in the classes you're already planning on taking.