4000 but they have limited to 3000.
That will surely account for lots of the spaces then. There were 3 rows at the bottom netted off, but that wont account to many at all as there were more in use and the top certainly has more in than the bottom - I'd say max 500 at absolute outside netted off, so 500 of the empty seats were always designed to be. Looking at the photo if there are 3,500 available seats, then there must have been 2,500 City fans (only 500 not used) which would mean roughly 1 seat in 3 empty and that feels about right (more space down, less up)
Edit - actually looking at the photo again, probably a few more but certainly I'd say 2,000+