Seven

The number of prime time network shows (I'm counting you CW!) that have used Peter Bjorn and John's (3 guys; my crack research team double checked) song "Young Folks."

They are:
Grey's Anatomy (ABC)
How I met Your Mother (CNS)
Gossip Girl (CW)
Las Vegas (NBC)
Dirty Sexy Money (ABC)
Big Shots (ABC)
Journeyman (NBC)

Wikipedia: http://en.wikipedia.org/wiki/Peter_Bjorn_And_John