6 cups a day is 48 oz of coffee. A travel mug and venti is 20oz. I don't know anyone who drinks more than 2 of those in a day.

Stop the hyperbole.

A travel mug is more an amorphous volume than simply 20oz.

A Stanley Mug, like the recent Stanley x Starbucks collaboration is 40oz. I believe this may be what they are referring to.

However, it's rare that individuals with the 40oz mugs are filling them with coffee, and if they're with espresso drinks, it's more likely that the volume of milk is increased rather than consuming 8 shots of espresso.

I'd assume most people with travel mugs that consume coffee are using 10 - 16oz mugs like myself?

Some Starbucks orders can compress 4 cups of coffee into one. The MG of caffeine in one of the larger orders can be enormous.