That leaves wiggle room in a couple of ways.
As of now, Atlantic Yards is supposed to feature 16 towers, Either a few towers would not be built--I'll give my bets below--or a few towers would be mostly or exclusively office space.
The residential mixed-use variation
There would be 6,430 apartments, at least in the by far most commonly described iteration: the residential mixed-use variation, as described in the Final Environmental Impact Statement. There would be office space in the flagship B1 tower (aka Miss Brooklyn), if built over what is now the temporary plaza.
Unlike apartment buildings, which can get construction loans (and tax-free financing) without committed tenants, office buildings need an anchor tenant, so the Atlantic Yards office space is on indefinite hold.
The commercial mixed-use variation
Alternatively, the commercial mixed-use variation would feature three office towers: not just B1, but also B2 at Dean Street and Flatbush Avenue and the tower across Flatbush at Site 5, now home to P.C. Richard/Modell's.
What towers might be missing?
While an office tower seems unlikely, it's not impossible, if the city wants to dangle tax breaks lure another Panasonic.
It's also possible, of course, to see a variation that differs from the two configurations disclosed in the Final Environmental Impact Statement. That document aimed to disclose potential impacts, not lock in commitments. For example, it said nothing about the use of modular construction, now under way. (Actually, shouldn't there have been some disclosure about the potential impacts of modular?)
But what if only 14 towers get built? There likely would be three towers on the arena block, excluding B1, six over the railyard between Pacific Street and Atlantic Avenue, assuming a deck gets built, and four over the southeast block, currently used as a surface parking lot.
That makes 13. The 14th tower could be B1, but, again, that needs an anchor tenant, and a tower there would disturb the plaza that seems quite integral to arena operation. (As I've written, Atlantic Yards would have had a much tougher approval process had there been no permanent office jobs to tout.)
The 14th tower could be the tower at Site 5, which is a fairly large footprint, though currently occupied by some busy stories.
Or the 14th tower could be Building 15, occupying a curious, 100-foot-wide slice of the block between Dean and Pacific streets just east of Sixth Avenue. That slice was initially supposed to be cleared out and used as construction staging as four towers were built simultaneously with the arena.
That didn't happen, so they didn't need the space. Part of the block is vacant lots and buildings, but there are also three private homes. Perhaps Forest City and the state don't want to kick people out of those homes. Or, perhaps, maybe they're waiting for the overall project, and associated construction, to make people more eager to leave.
The two configurations disclosed
According to the Chapter 1 of the Final EIS:
Two variations of the project program are under consideration to allow for flexibility in the program of three of the proposed project’s 17 buildings: (1) a residential mixed-use variation containing approximately 336,000 gross square feet (gsf) of commercial office space, 165,000 gsf of hotel use (approximately 180 rooms), 247,000 gsf of retail space, and up to 6.4 million gsf of residential use (approximately 6,430 residential units); and (2) a commercial mixed-use variation, which would permit more commercial office use in three buildings closest to Downtown Brooklyn and would contain approximately 1.6 million gsf of commercial office space, 247,000 gsf of retail space, and up to approximately 5.3 million gsf of residential use (approximately 5,325 units).