How many courses should I take in my PhD
Your success in a PhD program will be judged based on your research, not your course work.  You should take the minimum number of courses required to get the degree, and spend your remaining time performing research.

The exception would be if you are seeking a job at an institution where you would teach multiple disciplines, and are required to have taken a certain number of graduate courses in each discipline.  This would not be unusual at American community colleges.