It usually seems to be 12 plus 2 spares on long trials. I don't know if there's law behind that, but just something I've observed.
They can continue with 10 I believe but don't like to if it can be avoided.
I'd need to look it up but the heist trial had a lot of juror difficulties, although that was involving a gang. They got through so many it had to be abandoned and heard again iirc