Low voltage can also be the sign of a bad battery, bad cables/connection or poor grounds. If you have an ammeter check to see what amps are being produced.

Inspect, clean, tighten the cables and grounding points before deciding the alternator is bad.

If you don't have an ammeter then after doing the cable check and the voltage is still low, swap out a known good battery and re-check the voltage.

You can't have high voltage and high amperage at the same time. It's a trade-off between the two values.

High amperage is usually a sign of either a weak battery or a current drain (aftermarket stereo amplifiers are a common cause).

I wouldn't put too much faith in the plug-in voltmeter.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now