On Number of Nflation Fields

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.