What does "5 (or 10, 50, ...) public exhibitions" mean?

With the BASIC account you can have 5 concurrent public exhibitions. You can put into one exhibition as many artworks as the room has space for (between 20-45).

Also, you can publish/unpublish the exhibitions any time you want, the only limit is, that there can't be more than 5 ("BASIC" package) public at the same time.