JEE Main percentile and normalisation process have been prescribed by NTA (National Testing Agency), the conducting authority of JEE Main.

The formula to calculate JEE Main 2020 scores is:

100*( number of candidates appeared in a session with raw score equal to or less than the candidate/Total no. Of aspirants who have appeared in that session)

So a student scores 100 percentile means no other students score above him/her. He/she scores the highest mark in his/her session.

So the 24 students who scored 100 percentile this year means they are the highest scorer of their respective session. May be it's the Full Mark 360 or may not be.