I'd HOPE not.

Personally, I think I could say that it would depend on the composition of the prison population. if I was turning loose 10 innocents convicted of murder, but also 5 murderers, I'd have a lot more trouble making that call than if it was turning loose 3 innocents and 10 pot-smokers I don't know enough about the "average" prison population to really make a concrete decision in this regard, though. My best guess is considering everyone guilty has convicted crimes on "moderate" levels (like theft or dealing marijuana), I'd say around 70-80% would need to be innocent. But that's really just kinda pulled out of my ass. It would need to take more than a slight majority.

EDIT: that is assuming the innocents are accused of crimes on par with the actual criminals. If most were accused of murder and sitting on death row/life in prison and the actual criminals were petty theft and more mild assault charges, that number would shift downward.

In other words, I think that question has too many complications to give a simple answer