Ok, I think I now understand the codes:
J9 C9 D9 R7 I4 W4 E2 T2 Y9 B9 H9 K2

WTP has 3 fare codes (W4, E2 and T2) and I'm guessing you can't add those codes together to assume 8 seats for sale. So going by clubman it's actually only 4 seats - e.g. if x2 T's were sold then it would look like W2 E2 T0, with only 2 seats available?

The question then is by how many seats would BA oversell by?

Looking at Club there is either 9 or 11 seats available? I'm assuming they can't show a number higher than 9 per code so it could mean 9 seats or higher, so I've added R7 and I4 to get 11.

Thanks