I agree. Although of course if Swansea then beat Arsenal there will be another 1-2,000 tickets to sell I suspect, either i the south end of the East stand or in the usual half of the South stand.
It must have been more than that. They got the whole of the East stand (before it fell derelict) and possibly even the usual north east area normally reserved for away fans. Southampton brought 7,000 in 1985 and Arsenal brought more than them.
In 1987 they brought 9,000-10,000 in a crowd of 26,467. Surely Arsenal will want the full 15%* which is around 4,100 I think.
*Section 21, Distribution of Tickets http://www.thefa.com/~/media/776FDC2C663A48568FC3688FA667463A.ashx