How many lines are in a narrative poem?

It totally depends on the kind of poetry we are talking about. Haiku has three lines with five, seven and five syllables. Poems like Shakespeare's sonnets, or Grey's Elegy, that are written in iambic pentameter, have 10. Kilmer's Trees has eight. Limericks have 8, 8, 5, 5, and 8. The Night Before Christmas has twelve.

Some poems, like Beowulf or Ginsberg's Howl, are irregular.