What is the maximum wattage an iPhone X will support for wireless charging? The existing chargers on the market are all either 5W or 7.5W for their charging capacity.

While higher capacity chargers dont exist on the market, what is the theoretical maximum that an iPhone X can handle for wireless charging?
I assume your question is about the software support for wireless charging and not the potential capacity the hardware may support.

Initially all iPhone models that supported wireless charging (i.e. iPhone 8, iPhone 8 Plus, iPhone X) could only receive a 5W charge from standard Qi wireless chargers, regardless of how much the charger output. 

However, since iOS 11.2 Apple introduced support for 7.5W models and this is currently the limit.

It is unclear at this stage whether Apple can/will be able to further increase this limit if/when chargers supporting more than 7.5W appear on the market.