Does the iPhone 7 support 12w charging?

Charge your iPhone with an iPad or Mac notebook power adapter - Apple Support

Depends. My understanding is that the max input current with USB-A is 2100 mA, which translates to 10.5W. I've seen that when I connect it to my MacBook Pro. Under System Report's USB tree it indicates that it's 500 mA "Current Available/Required" plus 1600 mA "Extra Operating Current", as well as 2100 mA "Sleep Current".

I've also tried using a USB-A Lightning cable plus a USB-A to USB-C adapter on a newer MacBook Air. It says 2400 mA current available, which would be 12W.

There's really no way of knowing what exactly is going on without a current meter, and in any case the input current is often regulated to be short of the maximum.

I have a current meter, and when I had an iPhone 7 I measured 2 amps as the initial charging current when charging it with both the 12 watt iPad adapter and a 40 amp Anker adapter. At 5 volts that’s 10 watts consumed.

Remember that the current supply rating is what is available; the phone (or any device) will only take what it needs.

Lawrence Finch wrote:

I have a current meter, and when I had an iPhone 7 I measured 2 amps as the initial charging current when charging it with both the 12 watt iPad adapter and a 40 amp Anker adapter. At 5 volts that’s 10 watts consumed.

Remember that the current supply rating is what is available; the phone (or any device) will only take what it needs.

I suspect that what I see on a Mac may just be what it's willing to provision, even if it's more than the device will actually draw. When I've plugged in more than one iOS device I've seen the provisioning drop to maybe 1500 mA for one device and maybe 1000 mA for the other.

Yes.