I would guess 9 volts would probably start it. 10 volts definatly would.
9-10 volts equals 1 bad cell in your battery.
Voltage does not really matter, it is the AMPS that deliver the power to start the car.
All the stuff inyour house runs on 120volts, but the big stuff, like your blender, furnace, etc draw far more current (amps) at the same voltage. voltage=amps*resistance.
I don't know the exact # for req'd amps in your battery to start your car, but you should be ok w/ 12.5 I think. Check w/ the voltmeter when the car is running across the terminals of the battery- it should be reading 13-14 volts because it is charging.
If not, replace the battery or alternator. (at a shop if you are not sure even on how electricity works) Batteries are like 50 bucks.