During the academic year (Fall and Spring semesters), you must be registered for a minimum of 12.0 credits (4.0 if you are on reduced registration). The 12 required credits include both academic coursework and research credits, but the combination depends on your individual situation.
It is best to register for 15.0 credits in the case that you need to drop/withdraw from a course - that way you still meet the 12 credit minimum per semester.
Keep in mind - as you earn credits towards reduced registration graduate school only counts 9 credits per semester, even if you register for more than that.
General Registration Requirements: https://www.mines.edu/graduate-studies/general-registration-requirements/
Academic Calendar: https://www.mines.edu/registrar/academic-calendar/