Public worlds are constrained at the moment to the 1950-2007 band of years, and no. I don't know how many worlds there will be until it happens. Nor does stephen. Nor Balint, Nor lamp. nor carpet. nor desk.

we'll find out what happens when it happens. At the moment i'm thinking sufficient worlds to support around 3700-3900 players, as there's about 3200 purportedly active at the moment.
That should be sufficient, i hope, to allow for a natural progression from there as need be.