You need to connect all twelve pins to the motherboard.
You need to connect all twelve pins to the motherboard.
No, there isn't any. Most power supplies come with cables that split at the end, so plugging in half works fine—it doesn't matter if you connect all 12 or just 8, as long as you have at least 8 pins connected.
No harm in connecting them all, there's just not a reason they need to all be connected.