Some people are capable of managing multiple RPs at a time, some aren't. It depends on a lot of variables that are unique to you, not least of which is going to be the sheer amount of time you have available in your life to dedicate to constructing and managing your games. For myself, I have trouble managing more than one at a time because I am not an experienced GM and I also tend to construct very intricate plots with lots of details. It can be tough for me to corral my players and get them to move things forwards. Some people have an easier time because they are more experienced, or their games are not complex, or their players are more experienced and/or less troublesome to work with. I also work 60 hours per week on average and have a lot of other responsibilities in my life. Some people don't have that to worry about. You're the only one who can know what you're capable of. :)