How Many Auto Attacks Should an ADC Make Per Teamfight (on average)

I believe the answer you're looking for is "as many as possible." Try attacking the squishiest enemy you can safely reach without moving, weaving abilities between autoattacks and such, and build to do as much damage with as many attacks as possible.

How and when you build what items usually depends on what marksman you play, so I'd suggest doing some research on your favorites. Also, don't be afraid to build slightly different than the average marksman if you think it fits the character and you can pull it off.

"I need to land X basic attacks this fight" does not sound like the kind of thing you should be thinking about in a fight.

You should be the main sustained physical damage source in your team. I think it's acceptable if you were one of the last to die or survive team fight while always attacking and staying in position.

Three more autos against champions than the opposing ADC, and then 10-20 against a building or objective.

around 69 is good, however if u get 420 u win automatically

ADC performance isn't that simple, and sitting there counting autoattacks is a waste of energy.

ADC is all about positioning and dumping your damage on the right targets at the right time.

Enough to bring all 5 of the opposing teams healthbars to 0.

As many as you can without dying.