But those numbers measure Manhattan at its sleepiest, literally. Census figures count only residents, neglecting, as E. B. White famously wrote, “the New York of the commuter, the city that is devoured by locusts each day and spat out each night.”

If a whole city can be created and destroyed in a day, Manhattan comes close. During the workday, the population effectively doubles, to 3.9 million, as shown in a new report by the Rudin Center for Transportation Policy and Management of New York University. Day-trippers, hospital patients, tourists, students and, most of all, commuters, drain the suburbs and outer boroughs, filling streets and office space with life. Wednesday, it turns out, is the most populous day of the week, and special events, like the Macy’s Thanksgiving Day Parade, push the total past five million, offering a glimpse of what an even more crowded Manhattan might feel like.

So if Manhattan’s slow but steady growth continues — and there’s no sign it won’t — how many people can it handle? Answers to this seemingly simple question could fill enough pages to pack a spacious studio apartment, but a quick helicopter tour of future scenarios for Manhattan’s growth shows a tangle of towers and trade-offs.

Manhattan Circa 2030

Two hundred and fifty people work in New York’s Department of City Planning, and its Population Division, using various methodologies, tallies births and deaths, and then fine-tunes projections based on census figures, to arrive at a best guess of what the city will look like. By 2030, they expect Manhattan will have 220,000 to 290,000 new residents — roughly one new neighbor for every six current residents. They also anticipate a much grayer population, as more retirees choose to stay in the city.

All these new people will surely alter the skyline, and create demand for growth in pockets of the city that still have room to be developed. Certain neighborhoods come up repeatedly in conversations with planners and developers: Chelsea; the unnamed neighborhood some call “Riverside Boulevard”; the area near Columbia University; 125th Street; that stretch of parking lots on the south side of Delancey Street near the Williamsburg Bridge; and of course the West Side, where more than 18,000 new apartments are expected to be built in the huge Hudson Yards project.

Of course, in a city as seductive and glittering as New York, those projections could always turn out to be too conservative. What if a lot more people want in?

Let 1,000 Towers Bloom

These days, Ed Glaeser, a Harvard economist, inevitably comes up in conversations about how cities should grow. In his recent book, “Triumph of the City,” he makes an argument — which many consider persuasive — that dense places are uniformly better and more interesting than emptier ones, and that they should be allowed to develop unfettered, even if it means building towers where brownstones once stood.