Top 20 percentile cutoff marks for 2020 has not been declared yet. You can check last few years' top 20 percentile cutoff (out of 500) as reference -

Bihar School Examination Board, Patna:

For 2019

OC- 346 ; OBC- 347 ; SC- 332 ; ST- 339 ; PWD- 332

For 2018

OC- 293 ; OBC- 287 ; SC- 279 ; ST- 284 ; PWD- 279

For 2017

OC- 317 ; OBC- 312 ; SC- 302 ; ST- 306 ; PWD- 302

You can also check this link- https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx?i=File&ii=163&iii=Y