Looking at the base Jive Analytics tables (prior to loading into BI), the "Containers" table includes spaces, groups, projects and more. Projects are container_type = 600, but I don't see a way / method for identifying which projects are part of which spaces. And I don't see a foreign key / table relationship either. So when you figure this out, we would like to know.
The best way to contruct a nested query would be to use Container Name and Object Name in the Result Objects Box. Doing so will allow you to specify which container(s) you want to look into while giving you the freedom to use Object to look at other content types as listed in the Object Type Reference section of the Analytics Data Model.
In addition to Container Name and Object Name, add Container Type and Activity Count into the Result Objects box.
Next, place Container Type ID, Object Type, and Activity Type ID into the Query Filter box.
In the prompt window in the WEBI interface, type 14 and 700 for container type ID, 600 for object type ID, and 20 for Activity Type ID.
(14 is for Communities, 700 is for social groups, 600 is for projects, and 20 is for create)
The report query should like this:
Running this query will generate a report that calculates the number of projects that have been created in each space/group.
I hope this helps!