  We study the Nflation model, in which a collection of massive scalar fields
drive the inflation simultaneously. We find, when the number of fields is
larger than the square of ratio of the Planck scale $M_p$ to the average value
$\bar m$ of fields masses, the slow roll inflation region will disappear. This
suggests that in order to make Nflation responsible for our observable
universe, the number of fields driving the Nflation must be bounded by the
above ratio. This result is also consistent with recent arguments from black
hole physics.
