It all comes down to math unfortunately. With only 24 hours in a day (if you find a way to get more.....please let me know
) you need to decide how many of those will be used for school. Once you have a weekly idea then you can figure out your course load.
The average I've been given is for each credit, you should be spending 2-3 hours of study time out side of class per week (hence the math....4 credit class...8-12 hours a week). Now obviously, this varies from class to class (my 4 credit A&P1 class took far more time and effort than my 4 credit writing class)....but it's a good general guideline for a "Do I have enough time to add more credits?" reality check.
Also, a quick word of caution.....sometimes those little classes that we're sure we will blow through are very time consuming.....not difficult, but time consuming. I've had a few classes that surprised me that way.
Good luck with your plans!