Now that Apple does not ship a charger with the iPhone 12, you should think about buying a new charger rather than using the old one you have laying around.
Because your new iPhone can charge much faster than these old chargers can achieve. You should consider HomeKit Australia's 20W iPhone GaN charger. Modern iPhones all accept a charging input of 20w so there's no point using an old 5w charger. You will also need a 20w charger to get maximum output from any MagSafe device that you might plug in.
HomeKit Australia's 20w iPhone charger has TWO output ports, one for fast USB-C charging and a standard USB port for all your existing cables.
To check the output wattage of any charger, take both the chargers’s maximum voltage and amps and simply multiply them together.
You can't overcharge your phone so it is quite ok to use a "smart" 100w charger like the one supplied by HomeKit Australia and plug straight into your iPhone. Using our 100w USB-C to Lightning cable, the charger, cable and iPhone will all handshake and supply only the maximum power allowed to your iPhone.
Once you reach 80% charge, this power input will slow again. I charge my iPhone 12 regularly straight out of the 100w port on our HomeKit Australia GaN charger.
Still the fastest way to charge your iPhone (or Android phone) is via a direct cable to link to an AC charger. Wireless and MagSafe charging are slower. Another tip is for reducing charging time is to place your phone in Airplane mode if you need power quick.