How many activities am I allowed to join?

What are Challenges? How do I decide what type of activity to do? How do I decide which map to try? 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? What features are available in the non-physical version of the game? Once I have joined the activity, how do I know when it starts? and 5 more ...