Dear @riya
As you can read in this post the statistics of the second step will never be better than what you got in the first step of your simulation. So even if the number of primaries in your second step is larger than in the first step, the the reduction of the statistical error that follows is not real since you will just resample the same distribution multiple times.
As a crude estimate for the number of primaries needed in your first step simulation you could proceed as follows: run a full simulation (without the splitting into two steps) with only few primaries and then scale the number of primaries with the \sim 1/\sqrt{N} rule to get the desired precision of your results. But maybe someone with more expertise in this sort of simulation can give you a more helpful answer.
Best,
Lorenzo