Hi nismo, and welcome to our little corner of FT!

I assume you are using the phrase "B-Class" to refer to business class. AirTran's seat maps for business class are generally accurate (unlike coach seat maps). Go to their reservations page and find the flight you are booked on, then click on the seat icon. It will show which seats in rows 1-3 are already filled (again, don't trust this map for seats in coach). FWIW, the seat maps are updated in real time - if I have a wifi connection when asking for an upgrade at the airport I will request my first preference seat of those available; by the time my new ticket is printed and I reload the seat map page it shows my seat as now filled.

West coast flights tend to fill business class relatively early. If you want to upgrade using 4 A+ credits, you can do so as early as right after midnight east coast time (9:00 pm on the west coast) the day before your flight by calling. Upgrading with $ requires you to wait until 24 hours before your flight.

Good luck, and let us know whether you succeed in getting upgraded and how full business class was on your flight!