Full Idea
As soon as we start talking about 'possible world', we beg the question of their relevance to our prior notion of possibility. For all we know, there are just two such realms, or twenty-seven, or uncountably many, or even set-many.
Gist of Idea
We have no idea how many 'possible worlds' there might be
Source
Michael Jubien (Analyzing Modality [2007], 1)
Book Reference
'Oxford Studies in Metaphysics vol.3', ed/tr. Zimmerman,Dean W. [OUP 2007], p.99