We started out with the plan of allowing our three children to participate in three activities each but then we tried living the plan. Didn't work. Since some activities are scheduled 2-3 times per week, it became too difficult trying to manage everyone's school, work and play schedules. We needed to enlist the help of the grandparents to help get kids to different activities scheduled around the same times.
The only times our kids participate in more than 1-2 activities per session is when their school plans activities during lunchtime. Since they're already there, that's the easiest of all!