I'm doing 4 units a semester with 3 days a week work and 4 kids in school. It's definitely doable if you are committed but difficult to get great grades and fairly exhausting. Some units are easier than others so you might find 4 units works well sometimes and 1 at other times. My advice would be to try 2 and see how you go. Don't be afraid to increase or decrease your workload as the course continues. Good luck!