patient numbers required for clinical study , page-28

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Pardon my ignorance, I am also new to this,and i am quite happy to stand corrected but I am trying to reconcile your 1000 patients required for the trial, to that of the prof who said that orphan status can be granted to a company developing a drug treatment for a disease with only small prevalence in the population.
So if there is a small prevalance in the population, will the FDA accept less than the "several hundred to several thousand "
I acknowledge that a small prevalence in the USA may still be a large number.

Thanks in anticipation