can i charge my iPhone 12 max with an iPhone X charger?

Yes, but it won’t charge as fast as it will with a MagSafe charger or a 20W USB-C power source.

A nice feature of Apple’s world is that if the cable and plug fit it is safe to use it with any Apple device.

yes

It will not be as fast of a charge than the newer 20 watt charger Apple sells.