There is no way, it is a trade secret (hence why all airlines show 9 as a maximum, in fact some only show 4). The seat map is worthless as for many flights >50% of seats are unassigned for lots of reasons, or shown as "occupied" as a block reserved for various reasons such as codeshares; finally for longer routes not all seats may be available for sale due to load restrictions which vary with weather, routing and thus date of flight; so seat map occupancy has no bearing upon number of seats still available for sale.