I can't find an exact number online so you may have to pop into Vrigin or HMV and check the cd liner notes for writers credits but here's an interesting link about some of the solo and N'sync songs Justin has written.
Its known as 'change a word, get a third' (of the writers royalties). Many songwriting teams will allow recording artists to receive a writers credit in this way. It is the songwriters rather than the artists who receive the lions share of the royalties.