Blackstone's formulation

[bemildred]

5. As many as necessary.

If you can't get real close to that, I think you should take another look at your laws, they might not be good ones.

But anyway, this argument is ambiguous, guilt and innocence are decided by juries (ideally), and are also considered to be matters of fact, and in the second sense it is often never surely known what the truth of the matter is, that's why there are juries, to decide matters of fact, so you don't really have any way to assess the ratio in question, other than speculation and anecdote, and the record of reversals and such. So there is a false precision in discussing whether it should be 1:10 or 1:100, you have no way to know.

But you can say that no person innocent of the alleged crime should be convicted, it's almost mathematical. So if you have doubts about that, you have work to do to fix it. There is no OK level of false conviction or imprisonment.