Two instructors
During my PPL training I had seven, five of whom were also airline pilots working for the same company so we had to fit around their work.
The good thing about them was that they all discussed what they'd done with me with each other so I got the combined benefit of all their knowledge and discussions.
I agree about the idea of getting different perspectives from everyone you fly with too, this can be very useful.
As long as what's being taught is of a high standard and consistent it doesn't matter how many instructors you have, but contunuity is important...there's nothing worse than having a few weeks away and then having to spend most of the lesson re-capping what you did on the last flight.