Every single letter is a different fare class. Each has a different fare, and different rules. I know that on DL, T class is the cheapest/most restricted.

I have no clue as to actual numbers, just making it up here. Say you have a 50 seat RJ. You might sell 20 T fares, 15 U fares, 10 B fares, and 5 Y fares. So, if there are 40 tickets left to sell, all will show 9. If there are 30 tickets left to sell, T should show 0, U should show 5, and the rest 9, ETC.

This is by authorized tickets, not actual seats, I've not seen a single flight not authorized to oversell, but I've seen alot of fluctuation in how badly a flight is allowed to oversell. A few months ago, I was thinking about HNL. I pulled up the schedule, every single flight showed 9 in Y, I was happily suprised, I heared HNL was always pretty full. I then looked at each flight in detail, and every single flight was oversold. So, if its a big enough plane, it very well may be authorized to oversell by more than 9, thus making seatcounter kind of useless.