[TJR]

How many viewers does a series need in order to gain a second season?

Ratings are irrelevant for late night anime. There's no outside sponsorship (only the series investors themselves), and therefore no money made on broadcast.

Edit: I believe Fuji has even mentioned that noitaminA ratings alone can't determine if a project is successful. They seem rather concerned about the DVD angle lately.

Many factors other than ratings influence decisions about producing anime. Remember that most shows are intended to promote DVD sales as Enzo notes, and to encourage sales of the associated manga and merchandise. Sequels obviously also depend on the amount of source material available.

The measure of success also depends on the company involved. For instance, a manga publisher on the production committee mainly cares about increased readership and not DVD sales (consequently, they don't tend to care about show quality either, provided that it successfully ropes viewers into buying manga). The money they'd make from manga sales would greatly exceed the shared profit from DVDs.

Take light novel adaptations. Several have been successful on DVD (phenomenal even, in the case of Bakemonogatari and Haruhi), but the publishing industry isn't entirely thrilled because demand for the light novels hasn't increased anywhere as dramatically as manga.