What is the maximum adapter power to charge my iPad Air?

The Apple iPad Air models come with a power adapter that is properly suited to use with that device.

You can use the following support article as a guideline to what adapters came with what iPad Airs:

• About Apple USB power adapters - Apple Support

Like the others have already stated, a 20W USB-C adapter & charging cable should be more than adequate.

If your goal is to "fast charge" your iPad, you can use any Apple adapter, rated at 30W or more. Also note that you would need an iPad Air 3rd generation or newer for fast charging.

• Charge and connect with the USB-C port on your iPad - Apple Support

The "charger" is actually in your iPad Air, and it automatically controls the amount of current flowing into the iPad.

It won't hurt to use a higher powered adapter.....even up to 100 Watts or more.....but it won't really help either.

The phone will charge just as fast with a 20 Watt adapter as it will with a 100 Watt adapter.

there is no max really

personally I would not get a higher watt than 20watt as higher would just be a waste