And you can't ever really know how full the flight is from looking at availability displays.

For example, if an airline is thinking of cancelling a flight, it might restrict bookings in the way that you see on that flight even though it may only have taken a handful of reservations for that flight. This could be because the airline will happily sell a full or nearly-full fare ticket, and use the money to offset some of the costs of reaccommodating the passenger if the flight is later cancelled; but the airline may be unwilling to run this risk for tickets at lower fares. So it shows up as the airline only being prepared to take J class or C class reservations in business class, and only Y class and B class reservations in economy, with zeroes in every other booking class, even though there are actually very few (even perhaps no) bookings on that flight.

Conversely, I have been on flights where a day or two before travel, every booking class has had a 9 - and yet the flight went out 100% full. This can, in part, be because the airline can fill any space not sold at commercial fares with staff travel and other sub-load passengers.

One final thing to mention: the numbers are not linear. With J9 C9 showing, you could book one C class seat and find that availability immediately drops to (say) J6 C4.

So the only thing that you can ever really take away with confidence from an availability display is the number of reservations that the airline is, at that precise time, prepared to take in that booking class in one booking. Everything else is uncertain.