There are a few considerations, some of which have already been mentioned, that will determine how full or empty a section of the airplane is. I second what lederhosen said, that sales data will be considered state secrets, so you'll not be getting that information from an online forum. You're better to approach the airlines directly. With that, you'll have to tailor those airlines or segment them appropriately. What one airline considers first class, another will consider business class. So you've got a bit of leg work there to determine what a "first class" looks like, and who fits that bill. The target for first class and business class has moved in the last decade or two. My airline sells a business and premium section of the airplane, but those same sections 15 to 20 years ago would have been first class and business class for the services offered. Some airlines have updated their language, others have not, so there's another consideration.
But as to how full those sections are (whatever they're called), you have to consider time of year, business cycle, the conference and convention circuit, sporting events, and even day of the week. I can tell you at my airline that our business class is full almost every flight. When it is not full, its generally because we're going to a sun destination in the shoulder seasons when we only have half the airplane filled, and moving everyone into business would create a weight and balance issue. This happened to me just last week. In other cases, it's because you're flying between two business centres mid-week or early weekend when it's generally families travelling. In almost every other case, the section is full. What the breakdown is between those who paid full fare or flex fare, those who were bumped and upgraded from an earlier flight, those upgraded because of status, or those that are airline staff is outside the information delivered to me in the manifest. I can if someone is a status member or an employee, but I'm not told how they paid for their seat or if it was an upgrade. I don't search for that info because it's not required to complete my job.
But, I can tell you that if the section is not filled by fare paying passengers, it will be filled by upgrades to give the impression that the section is desired. So upgrades are handed out quickly and quietly at the gate or even before. As far as upgrades go, each airline is slightly different. Generally, upgrades are only given once all passengers have checked-in and quite often no earlier than an hour from departure. Any earlier than that, and the section was almost empty. Then (at least at my airline) upgrades go in priority of passengers voluntarily bumped from an earlier flight first, then flight status passengers, then the standby list in order of priority (employees, employee companions, contractors, buddy passes). At some airlines the pilots and flight attendants have language in their contract to deadhead in business where a seat is available, and where they slot in will differ with each airline. We don't have that language at my airline (fingers crossed for next contract). But there are as many ways of upgrading passengers as there are airlines, so it might be hard to pin down an "industry standard" in that regard.