How many Team Magma/Aqua Pokemon need to be in play in order to attack with Team Magma's Groudon EX or Team Aqua's Kyogre EX with the "Power Saver" ability?
Five. If there are only four or less, it can't attack. But keep in mind that it counts Team Pokemon on both sides of the play field, including itself.
Source: TPCi Rules Team (2015-04-23)