I just upgraded to the 8GB ram limit, though this would apply if someone were on the 1GB free plan limit as well.
How is this limit computed when I have multiple apps and multiple users on an app? Is it 8GB across all apps and all users? 8GB for each app, across all users? 8GB for each app and each user?
Each of the account's applications is able to utilize 8 GB of memory for each instance.
The number of users an instance can support is a function of how many workers and how many connections per worker the application is configured to use.
See the Advanced Settings section of the documentation for more details.