How many activities am I allowed to join?

How do I decide what type of activity to do? What types of Activities are available? What are On-Demand Activities and how do I join them? What is a map? How do I join an activity physically? How do I join an activity non-physically? Once I have joined the activity, how do I know when it starts? What are playlists? Can an activity have a maximum number of players? Can I save items as favorites? and 2 more ...