In theory, the evidence of one officer is enough and it doesn't matter what the crime is.He may be obviously truthful and accurate and the defendant may be equally the opposite, so the jury is sure of guilt.However, the standard of proof is very high in a criminal trial and the burden on proving the case rests on the prosecution,
so the court is unlikely to find the case proved when it rests entirely on one man's word against another's.If there's no evidence otherwise that any crime has ever been committed then plainly the jury is unlikely to convict.
.In practice juries tend to accept what any officer says he saw more readily than they'll accept his evidence of a confession (unless, of course, the confession is recorded on tape).It is sometimes manifestly unlikely that someone who has done a crime is going to admit it to a single policeman and then go on to deny involvement soon after and evermore.
A policeman, even just the one,describing the circumstances as an eye witness and giving details is better for the prosecution than one claiming to hear a few words of confession , if only because every detail can be examined and tested thoroughly by the defence.If the account holds up then, the jury may be satisfied that it's true. A confession may end up as a case of 'he said it' and 'no he didn't' without much more.
Some admissions and confessions fall into a special category. These are ones where the person confessing is the only person who could or would ever know particular details of the crime or even that it was committed.It does sometimes happen that a man walks into a police station or up to a policeman and says 'I've just murdered my wife' which is the only reason the police then know of the crime and go out to investigate, finding the corpse. That confession is very hard to challenge!
Basically, an officer may arrest anyone if he has reasonable suspicion that they've committed a