If buying new you'll want to get a psu with the proper amount of connectors
If you already have it, it has enough power
For a single card system EVGA reccomends;
Minimum of a 450 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 24 Amps.)
Two available 6-pin PCI-E power dongles
There's no OCP on an M12II's "dual" rail's, it's a single rail psu with 2 times as much 12v power as is reccomended for a system with a single card, that reccomendation isn't doubled for dual card's