How many layers do the Boeing 787 cabin windows have and what might make them fail?
This article about the Boeing 787 Dreamliner states that


  [...] The Dreamliner’s new windows are made of two thin layers of glass with gel wedged between them. [...]


The windows have also a failsafe mechanism in order for the outer pane to crack or fail first, it's this tiny hole that you may have noticed, which serves to regulate the pressure exerted from the cabin to the window.

I couldn't find a more technical article about aircraft windows, but I found an AIAA article about Space Shuttle's cockpit window system, if you are interested about something a little bit more extreme!