People often consider it a holy number. For instance, there were 12 tribes of Israel, Solomon had 12 officers and Jesus had 12 disciples. However, other variations are possible. Greek juries were huge - there were 501 jurors at the trial of Socrates. Only six people sit on the criminal juries in the Territories.
Most people today think jury trials are a great thing.
No, only force could convince many prisoners to accept jury trials. Conviction meant death by hanging or burning, and some feared the jury was biased. Also, many accused people were afraid for their families. A guilty verdict would mean they had to give up all their property to the king. When they refused to consent to a jury trial, the courts used painful measures to try to force their consent.