With dual full-time working parents we are forced into the one activity per child at a time, maybe two. Personally, we wouldn't do more than that because it would be very hard. BUT, I respect your wanting to introduce your child to multiple activities. Last year, we spent the school year in gymnastics and while my daughter has been begging to go back, if we wanted to introduce her to soccer there would be a conflict. So we skipped gymnastics and started soccer. But now that soccer is over, we hope to go back to gymnastics again (until softball starts). Because we limit the quantity of activities in a given time frame, I'm less concerned about overscheduling and more concerned about how you really develop good skills. It takes time to do this and if you keep switching around, how do you know what your child could really be good at?
I think in another year or two, my child (who just turned 5) should hopefully be able to pick one activity a season to stick with (sort of like it happens naturally due to sporting schedules. Do what your family can handle in terms of timing but also keep in mind how your child is reacting to all the different activities. Each child is different.