60-70,000 according to bbka study notes module 1.
So much for the study notes, unless this result has been taken out of context for the question here.
The deep box has around 50 000 cells, so an estimate of 60 000 indicates only a 20% increase in area, so the lower end of the range must be a load of clap-trap for a start.
Further, the 'standard' box was calculated on the standard 11 deep frames - and 12 are quite possible in the 14 x 12.
The correct answer could be around 77 500, and will be over 70 000.
The result from the study notes may be taking into account 'honey arch' and things like that - I don't know and frankly don't care - but quoting 'like for like' is a good basis for comparison, not being wishy-washy and giving a range of plus or minus ~8% for a mean which is patently incorrect.
A simple calculation, pro rata, for foundation sizes (which is not quite accurate but will suffice as an example) is:
12 /11 * 50 000 * 11.5 / 8 = 78 410
RAB