What is the maximum current and voltage that can be charge iphone 8 plus safely?

Any properly designed USB charger is OK.

A standard USB charger, such as the iPad charger, can provide 5 V at up to 2.4 A.

USB-C chargers can provide up to 20 V at up to 5 A. They always start at 5 V and only use higher voltages if the device asks.

The current draw is controlled by the device, up to the charger's limit.