IC: Historical Rome using 5e rules.
To be honest running several games it is not as demanding as people might think, the setting up is the most demanding part, you must read a lot to set things properly, once the game is running you only are required to read a bit here and a bit there to post the next thing happening. I love PBP because you are able to take time to read before resuming the storytelling, solve doubts, research rules, etc. Stopping the game in a live session to read and research would make your players yawning and getting impatient.
I have 8 games now and opposite to what people might think I am answering a couple private messages per day and moving from 4 to 5 posts per game at most, some games even don't move in days so I do not see the problem in handling many games at the same moment.
Another variable is my job, I do not spend time traveling to job, my job is near home and I also have in my working time some minutes here and some minutes there to check the site and answering messages and posting in the forums, a privilege not many have.