- Driveway of house: carriage drives by - 90 crew
- Back lawn: Lizzie watches Mr Darcy gallop up to house - 90 crew
- Main stairs: Lizzie agrees to dance - 30 crew
- Ballroom - 50 crew, 150 extras
- The Bennetts leave the house - 90 crew
The word "required " is of course used loosely. I think we can confidently analyse the situation as follows: it would not be plausible to claim that 90 people fit on a staircase, therefore a random number of 30 is chosen. A ballroom, being bigger, can plausibly accommodate a few more - on top of the generously-stocked company of extras. However, any number of people can fit into an outdoor location, so the full complement is allotted whenever the open skies beckon.
You can bet the budget reflected this generous allocation of manpower, even if rather fewer than 90 people were actually, errr, working. A striking reminder of the days before the Times moved to Wapping and put paid to print unions stuffing Fleet Street payrolls with individuals such as D.Duck and M.Mouse, all of whom were said to put in full shifts every night - and certainly drew full wages.
This is what happens when tax-breaks are awarded to a favoured sector of the economy. The filming schedule cited above was from 10 years ago, but did the financial crisis change things fundamentally ? Still some modest scope for cost reductions in this particular 'industry', I should think.