"Headphones" is plural. The "phone" bit refers to the actual drivers, of which there are at least two in a pair of stereo headphones. That's why we say a pair of headphones.
With regular AirPods, they work individually. You don't have to use both. So it's helpful to think in terms of each one being its own thing, even though they're paired.
I'd add that over-the-ear (so, one item) "headphones" have a similar issue as AirPods Max. I'd probably call two of these products "two pairs of AirPods Max." (Well, actually I'd probably just say 'two pairs of headphones' but that's a dodge.)
In context they treat Airpods as plural, using "are" over "is". It makes sense because the originals were plural, separate objects.
>I saw two AirPods Maxes
It is a compound noun, the base word gets pluralized, so it would still be Airpods Max. Like the plural of cup of tea is cups of tea, or sergeants major, brothers-in-law, courts martial, passersby. Airpods would be the base word.
AirPods is the product line name. Max is the model. So you have AirPods, AirPods Pro, and AirPods Max. If you needed to buy 10 of these you would say your were buying 10 pairs of AirPods, AirPods Pros, or AirPods Maxes.
Honestly though, it's English we're dealing with, so there are more exceptions than rules, anyway. Muddy the waters further with proper nouns and branding considerations and basically the only people who can tell you what is "right" would be Apple's marketing dept.
The AirPods Max is a headphone.
AirPods Max are headphones.
I saw two AirPods Maxes.