Most "midterms" given by law professors are practice exams. I had all different types. Sometimes the prof gave us the exam, which was a shortened version of an old exam, let us work on it for 40 minutes, then went over it before class ended. Another prof gave us a whole class, then discussed it at the beginning the next class. A third prof would reserve the room, and give a practice exam to make up for classes she had to cancel. None of these counted toward our grades, they were just to track how well you understood the course. My contracts class had a midterm. That though was because it was a year-long course, and the school wanted to keep the students honest. My contracts midterm will be used to "adjust" our grades on the final. Basically if you did a lot better on the midterm than the final, your grade will go up by maybe half a grade. And the opposite if you did a lot worse on the midterm. Other contracts class made the midterm a certain percentage of the final grade, like 25 or 30%.
Generally though, you will have one graded exam per semester.