This is definitely something we have on our radar for the future as Ghost gets more complex features & use cases.
The current expectation is that Ghost would be run on a single instance, with a cache or multiple load balanced caches in front of it to handle scaling.
The reasoning is that for most cases, it’s the content that needs to be highly available, not Ghost or the Admin panel. Even serving slightly stale content or API responses is usually acceptable.
We 100% realise that over time, as more people depend on the API and so on, this will change and so it’s something we’re thinking about now, with a view to slowly refactoring behind the scenes as opposed to it becoming a project all on its own.