I focus on one at a time. No matter the genre.
However, depending on my academic period, I tend to play certain genres. During the period of exams, I try to avoid long games with complex stories (such as JRPGs) and stick with more "arcady" games that I can play for short periods of time and don't require too much investment from my mind (like rhythm games, platformers, puzzle games...).
During less busy times, I try to tackle those long games I had to put aside.