There's no ideal number. No matter how you do it, innocent people will be put in imprisoned and guilty people will go free, so you're just making an arbitrary trade-off by defining a limit. Though I like the way it is in the US: you can only be tried once for one crime (except for the civil suit when there is one).

I heard the law was to prevent some sort of corruption/exploitation, but I forget exactly what. I think it was to prevent people/the system from being abused for political reasons. I lean toward mercy when it comes to law enforcement and punishment, which is why I prefer it to be one time. Better not to have something hanging over you your whole life (or as long as the statute of limitations lasts, I guess) and better for 5 guilty people to go free than for 1 innocent person to be imprisoned or even executed.