In views, show the number of users who have subscribed to the organic group
This is how to get the OG membership ID:s to start with.


Add new view. Show -> OG membership (important!)
Then in some display add a contextual filter -> OG membership: Entity id. 
Provide default value -> Type -> User ID from logged in user. 
Override title: %1's groups. Specify validation criteria -> User.


Now you have the list of ID's.


Next to get the actual titles of the groups (as the base table is OG membership, not node), Add a relationship -> Group Node from OG membership.
Add a field -> Content: Title. Relationship -> Group node from OG membership.


Now you get the titles too. For test add some filters and sort criteria.