In many cases it’s unclear. In VA McAuliffe tried to restore voting rights under his power as Governor. The VA Supreme Court short down him trying to do so in one motion, so he had to sign off one at a time until all 150k or so ex-felons had there rights restored. I believe Charlie Christ had to do similar in Florida.
As far as I know no one has tried a blanket pardon, I expect a similar standard of individual sign offs to do so. This would give the senates time to impeach before to much nonsense occurs.
Andrew Johnson pardoned huge numbers of former Confederate officers, soldiers and public officials - although not all - without individually naming them. Gerald Ford pardoned thousands of Vietnam draft dodgers and deserters, if they turned themselves in, reaffirmed their loyalty to the United States and did two years of community service (Carter’s mass pardon of draft dodgers, but not deserters, was “no strings attached”).
The President has very broad pardon powers and its limits are still untested. A pardon of, say, all Federally-convicted murderers could conceivably be seen as an abuse of power, and a high crime and misdemeanor warranting impeachment, by Congress.
Under what grounds could someone go to Federal court and get an emergency stay on the action? It seems like anyone who could show faced a real risk of being murdered by these newly released murderers could get a stay, similar to how Trump’s action in banning people from certain countries had a stay put on it.
The stay is so they don’t have a flood of released prisoners before Congress can even discuss the issue. Presumably, the Supreme Court would uphold Trump’s right to do this, but they have to pick up the case and vote on it.