I'm doing my 2nd-session prep, and the book doesn't say anything on how many Fronts is good to prep. I'm not too worried about it--I'm going with two, with a total of 7-8 threats, and I don't imagine the players will tear all that down in one session.
This brings up the question, though: How many Fronts are usually assaulting the players/being assaulted in any given session of your games? Is one Front too few? Three too many?