There is no effective limit on pardon power. The most famous example of mass pardons was Pres. Andrew Johnson “Proclamation 179”, a mass pardon for treason arising from the Civil War. Jimmy Carter’s mass pardon of draft dodgers was another.
i was thinking of state powers-mcauliffe screwed up in virginia with blanket pardons, i believe they have to be done on individual basis.
https://reason.com/blog/2018/04/18/gov-cuomo-to-use-pardons-to-restore-voti