Sold out is a bit vague as within limits City can decide how many season tickets they are going to sell. Sold out just means when they take the off sale, and they use the term waiting list and sold out to make it look like there's big demand.I see no reason for the club to say they are sold out if they aren't. There is still plenty of time before the start of the season to shift many more, so stating it as sold out doesn't make any business sense unless season tickets actually sold out.
I don't suppose anyone has any numbers? Last thing official I know is 40,500 season ticket holders (Report and Accounts) for the 2015-16 season, and that record was broken last season, but as far as I know they never said how many season tickets were sold. I have a figure of 42,000 but I don't know where that came from.
I've heard we also have 2,000 corporate sales so assuming they are not in the season-ticket sales then I think 'season tickets' are around the 44,000 mark.