The problem is when I was in high school, I would have people come through with "one" item broken into 48 parts. For example, 48 cans of cat food which are individually sku-ed by flavor. So, it's not "one" item.
For me, I honestly consider the currents lines and actually work involved in scanning what I have. For example, if I have 4 gallons of water, only one needs to be scanned to capture all 4.
But if I have multiple sets of items like that, I am not likely to go into the express lane.
Interestingly enough, in many stores, the staff is not allowed to kick you out of line for "quantity" violations. It's usually other customers that are going to call you on it. I've been in stores before where someone with a nearly full carriage uses the express lane because their time is too important to wait in the regular lines. Unlike the rest of us whose lives are so uneventful, we want to wait behind their huge order so we can pay for our two items.
And I imagine for a purist that the answer is simple: most receipts have an "item total" displayed clearly somewhere on the receipt, and the numbers don't lie.... that is how many items you had no matter how you justify using the express lane.
