Five at the moment. The personal/side projects (2 of those 5) get worked on whenever I have the free time to. The others, I have dedicated days of the week that I work on and am available to work on them, e.g. one of them gets Mon./Tues., one gets Wed. and one gets Thurs./Fri.
That way I can dedicate enough large blocks of time to the projects to really get some good work done and my clients know exactly when to expect to be able to reach me and when to be getting updates from me.
Of course, every now and then emergencies or major deadlines happen, but for the most part, I try to set a schedule and stick to it.
