Due to less no. of students appeared in jee mains this year and also increased competition (since corona gave extra time for preparation) , how can the cutoff rank (closing rank) for colleges can change ?

Due to this pandemic, students got a lot of time for preparation and results 24 students getting 100 percentile. So this is true that this year there is a tough competition for councelling in JOSSA. If you want to know what are the expected opening and closing ranks could be in 2020, so just subtract 70 in each opening and closing rank and you will see where can you stand. I'm saying this because if you prepare for worst then you can get best.

Here is the link for last year opening and closing rank for admission in engineering colleges,

https://www.google.com/amp/s/engineering.careers360.com/articles/jee-main-cutoff-for-top-nits