When creating your calculated metrics you can use the system metrics, user metrics, activity data, and custom data on company profiles.
These are all items that will be specified as the "prop": value, and here is how to ensure you're using the correct naming conventions.
**Always, when creating a calculated metric and specifying which metric/activity to use, use the EXACT spelling, spacing, and capitalization that displays.
However, when creating calculated metrics from the System Metrics, use the following:
system.users_total (for Number of users)
system.activity_count (for Activity Count)
system.users_active_last1 (for Users active last day)