Rule *254 - Sessions Of Jury Trials(a) Except as provided in subsection (b), juries in those civil cases which are for trial by jury shall consist initially of eight members. Trial in such cases shall continue so long as at least six jurors remain in service. If the number of jurors falls below six, a mistrial shall be declared upon application by any party.(b) Trial by a jury of twelve members rather than eight may be had if demand therefor, either by endorsement on a pleading or by a separate writing, be made by any party not later than twenty (20) days after service of the last permissible pleading.