Determining a “maximum” depends on what you consider to be the limiting factor.
I don’t see a problem with PostgreSQL
PostgreSQL: About
Maximum Table Size 32 TB
Even with optimal indexing, query speeds will at some point take “too long”. What that is will vary from person to person.
Then there’s Ruby, Ember, bandwidth and browser limits. Feel free to research, but I wouldn’t worry about those.
I think that long before any software limits are reached “sane” limits will have already been passed. And what that would be will vary from person to person and how they interact with the information.
tl;dr You will break your users long before you break any code.