Stage 1 is the first part out of four in SASUKE. It is always the easiest of the entire contest, which is why it is in the beginning.
100 people compete in Ninja Warrior, and that means that 100 people compete on Stage 1. Stage 1 acts as the sifter of SASUKE; all the unserious, attention-seeking, unfit people are put through, and only the real athletes who have promise make it to Stage 2. It varies, but the average number of survivors can be from 20 to 3.