Can too much wattage on a charger destroy my iphone 7 plus?
As long as the charger is standards compliant, your phone will not be damaged because of the wattage. The phone and charger makes sure that the phone does not receive more power than it can handle.

The 50W charger you saw on Amazon is most probably misleading you. There's a bunch of these advertised as outputting "up to 50W". The reality is that the chargers have multiple outputs and each output only supports up to 2.4A per port - which at 5V is 12W.