How much voltage does a 9-volt battery hold?

9 volts (that's what the 9 means)

Further Explanation:

This sounds like a silly question, but a battery is not always at its' nominal voltage. In general, non rechargeable batterys of the same type will have a higher terminal voltage than rechargeable. Over time, as the battery becomes used, its' voltage will sag until it is no longer useable.

Rechargeable battery cells are usually rated at 1.2 volts. When fully charged, they will be slightly more than this; as they age, they may become less. For cheap rechargeable 9 volt batteries, the manufacturer might use 7 cells - 8.6 volts; others might use 8 for 9.6 volts. Non rechargeable usually have 6 cells rated at 1.5 volts each (6 x 1.5 = 9).