The Enterprise plan allows you to use Rails LTS for up to 20 applications.
A monolithic Rails app counts as a single application, regardless of on how many servers it runs, how many (sub-)domains it serves, or if there are staging or test environments.
If instead of a monolithic app, multiple Rails apps are used in a service architecture, but look like a single service or product to the outside world, they still counts as a single application.
Apps with different codebases powering separate services or products count as separate applications.
An agency maintaining applications for several clients can use a single Rails LTS license, as long as the application limit is not exceeded.
If a client also has its own developers working on the application, a separate license is required.