We can't be sure about the greeks, and I still don't know how well it scales personally. Memory champions claim to have quite a lot of palaces and ways to cram information in them (for example, for memorisation of digits of pi), but my personal experience is not that big as of yet.
As a matter of fact, an informal list of places I can use as memory palace, as I know them from memory ranges around 70 "palaces" with between 4 and 15 rooms each. If you cram 3 items per room its around 700 things to remember. But you can get better.
You can also use association (I am about to write a post about it) to get facts about books, people, things without using a new palace for each kind of data type.
When he writes about this sort of thing Derren Brown (British mentalist) mentions that once you get to a decent level of skill with this you can just make up new rooms in your head; he talks about "dropping" overused rooms and starting fresh ones.
As a matter of fact, an informal list of places I can use as memory palace, as I know them from memory ranges around 70 "palaces" with between 4 and 15 rooms each. If you cram 3 items per room its around 700 things to remember. But you can get better.
You can also use association (I am about to write a post about it) to get facts about books, people, things without using a new palace for each kind of data type.