Generally speaking, you will need to register for at least 12 graduate credits per semester until you have 24 graduate credits. Beyond this, the minimum requirement is 9 graduate credits per semester. Note that international students on student visa must register full time. Also, note that these credits must be graduate credits and not undergraduate credits. Courses with numbers 500 and above are graduate courses.
In the semester you graduate, you can do less number credits by getting permission for under-load and still be counted as full time. For more details see the Graduate Handbook.