156

156 is the chord progression playing during the chorus. the chorus is in c# minor, so when they sing 1 5 6, the chords are actually changing on the rhythm that 1 5 6 is sang-- c#min G#min Amaj. although the bass doesn't play the root of the 5 chord, which is G# minor, the 5 chord is very strongly implied in the instrumentation and in that synthesized string line which plays G# (the root of the 5 chord) right when they say "5". i dont know if this is actually why they say 156, but if it's just a coincidence that they're saying 156 and the chord progerssion happens to be 156 changing right in rhythm with the vocal line, then damn that's quite a coincidence.