Won't start or won't run?

Once it's started (assuming the alternator is working) it should run with a battery as low as 10 volts. Though you'll likely have to jump start it if the battery is lower than 12 volts. But some will start even with 11V.

However, why is the battery low?

If the battery is no good, the voltage will likely drop considerably when put under load. (eg. 2V)

If the alternator is not working, your battery will naturally run down. And then without aid from the alternator, all but the really old cars are likely to have problems even after you jump them.

If the battery is good but a bit low just from sitting too long, you should only need to jump it.