How many walt does iphone x could be charged? the maximum

Apple never specified exactly how fast "Fast Charge" is.

I believe that iPhones directly connected to USB 3.0 ports on Macs report 2100 mA current reserved, which is 10.5 W. I have connected an iPhone to a USB-C port on a Mac using a passive adapter, and that reported 2400 mA (12W).

The "Fast Charge" for iPhone 8 to iPhone X models has never been specified by current or power. The tests I've seen suggest that it's maybe 15W with a USB-C Lightning cable and a sufficient USB-C power adapter.

You can use up to an 87 Watt charger, but your phone won't charger faster with it than it will with an 18 Watt charger. Thus, buying anything faster than an 18 Watt charger for your iPhone would be a waste of money.