Hello kshama.

From 2019,NTA is conducting JEE mains and it is being taken in a number of days for different students.So about the cutoff thing,they are decided taking into consideration the number of students that have to be selected for the next round.So if for example 1,80,000 students are to be selected then out of the students appearing for the examination the top 1,80,000 students are selected and the lowest rank person's marks determines the cutoff marks .Similar process is used for category wise cut-offs.

Hence,they should make a overall result of all the exams taken,be it on any day and then select the top required students and eventually which will determine the cutoff.Hope this was helpful.

Good Luck!