I grew up in Australia where a jury consists of 12 people. I've lived in France for the last 12 years, though, and here the jury (when there is one) it will typically be either 6 or 9 citizens. One of the overseas territories uses a jury system that only has 4 people, and then expands to 6 on appeal. It would be interesting to understand how the 'Anglo Saxon' system (outside of Scotland, it seems) settled on 12 people.

It wouldn't surprise me to find it's based on the twelve apostles, that Jesus Christ used as his witnesses.