A good indication is the ticket criteria:
Season Ticket holders with 5+ away credits from the 2015/16 season from Thursday 21 January at 8.00am.
Season Ticket holders with 4+ away credits from the 2015/16 season from Friday 22 January at 8.00am.
If tickets remain a further announcement will be made on Monday 25 January at 9.00am.
Whenever they say "if tickets remain" that usually suggests that they don't expect tickets to be left after that point. So I reckon they're expecting it to sell out to STH with 4+ based on the info they have on number of matches attended by each fan.
Yeh 3,600 have been to 4 or more aways (including homes)