as many as you feel you can handle so that you do well in all your classes and pre-requisites. but typically ppl do at least the minimum for being a fulltime... iono wat it is for the semester system but for the quarter system it's 12 units minimum each quarter to be considered a full time student.
I actually think they do look at your course load per semester. If they see you're only taking 12 credits without any due reason then it'll make you appear lazy. I think minimum should be atleast 15 credits per semester unless of course you have valid reasons such as work or a child etc. Otherwise, you should stack it up.