Gregory's Server
Simplest answer: Only posts your instance have seen.
The question is, what are the methods your instance is using for its federation?
For example, there are relays out there which push posts to instances subscribed to it.
There are also setups wherein an instance fails to receive posts from another instance because they have a configuration that is preventing them from fully federating (even though both admins claim they haven't silenced or blocked each other).
I am personally not aware of any #fediverse software with a built-in code that searches the "entire" fediverse network. I think that would be very taxing. It's like being a search engine at that point.