I think it just depends on your child, and how much time the activities actually take up per week.
Our daughter just started kindergarten. She is doing gymnastics once a week (45 minutes), which she's been in since turning 3, and she just started dance because she wanted to (ballet, then tap, back-to-back on the same day once a week). She is doing 6 weeks of Saturday morning-only soccer, and has 4 weeks to go. To me, this is plenty, but she wants to do even more! She keeps asking me when we are going to do swim lessons again and when she can start piano! And I'm like, hey kid, take it easy, slow down! But that's her...she's always been high-energy and loves everything she tries, so I imagine as she gets older, it's going to be hard for her to pick and choose. But I would like to see her do music, plus at least one physical activity of her choosing (dance, etc.).
Our friend's 6 year old daughter is in 1st grade and does soccer (2 practices plus 1 game per week) and dance (3 classes per week, 2 on the same day). She's high-energy too.
You know your child best, and what you think they can handle. If she's not that high-energy, she might do better with something like piano if she's interested, but I would still want her in some kind of physical activity just so she can get some regular exercise. It does not have to be competitive, but more about building a skill, like swimming, and keeping her active.