According to Parris, what will happen if Hale can get a confession from one of the prisoners? What might happen if Hale cannot get anyone to confess?

If Hale can get even one confession the town would believe them all to be guilty; if he doesn't, there will only be doubt.

Parris: Now Mr. Hale’s returned, there is hope, I think—for if he bring even one of these to God, that confession surely damns the others in the public eye, and none may doubt more that they are all linked to Hell. This way, unconfessed and claiming innocence, doubts are multiplied, many honest people will weep for them, and our good purpose is lost in their tears.