I don't think that we'd sell out as STs +1, but I agree with you, and I reckon there would be an almighty binfest if anything other than 1 ticket per STH or member was announced. You only have to look at this thread to know that whatever form of prioritising for allocation is announced, people...
23K season ticket holders, of which a good 10% won't be going for whatever reason, so that leaves about 13-14K for everyone else depending on allocation. I'm failry sure anyone with a purchase history this season that wants to go will get a ticket.