Greetings aspirant!

There are total 23 IITs participating this year. Seat Matrix for 2020 hasn't been released yet by the authorities. You should know that seat number will vary from iit to iit and also from subject to subject. For example :- suppose it's iit delhi and subject is chemical engineering then the 2019 seat matrix for SC was 11, and if the subject is civil engineering the the seats for SC was 17. You can yourself check seats Matrix for previous years through this link.  https://engineering.careers360.com/articles/iit-seat-matrix

Hope it helps!