I think its going to be more than 5 songs. They might do one new song and one classic song every night.
Also, it should be noted that this is the same week that Jimmy Fallon replaces Conan on the Late Night show, so perhaps U2 will be performing as a bridge to the Craig Ferguson show to prevent people from watching Fallon's debut week.
Future performances are too far away, but I can see U2 doing some additional promotion when their tour starts in the U.S.
And I think this is one reason they are doing some heavier promotion in the U.S. now as the upcoming tour, unlike most of the past tours (since JT, excluding Love Town, which never hit the U.S.), is not starting in the U.S.
While it remains to be seen, I can envision Letterman talking to U2 more on the first night, meaning U2 may only perform one song then.
On the other nights, I can definitely envision two songs. And I think U2 would be wise to play a good combination of old and new. Fans won't know the new material, so lure them in with a classic, then, hopefully wow them with a new song. Rinse and repeat the process for the week. If Letterman talks to U2 on the other nights, it probably would be far shorter (after all, how many times can one ask about the "new album" or "tour" or DATA or the history of U2, etc.?).
I'm glad to see this promotion. As I've mentioned in other threads, despite U2 having multiple #1, Grammy winning albums that have sold nearly 60M copies in the U.S. alone, despite having numerous Top 40 hits and plenty of other Hot 100 hits, it seems radio stations just don't play U2. The powers that be on radio seemed to have deemed that Clarkson, Kanye, Beyonce and Timberlake are all that we should hear.
So U2 have to resort to doing some sort of promotion to get the word out. I'd rather see 5 nights on Letterman than a commercial, as this way no one can complain about "selling out".
I don't watch Letterman (can never stay awake that long), but I certainly will be tuning in that week! 
