It’s not just what’s inside that counts.

While some 20,000 Donald Trump supporters filled Madison Square Garden to capacity on Sunday, many thousands more were turned away, according to law enforcement sources.

Estimates on the number MAGA faithful in Midtown Manhattan varied, but one cop on the scene guessed that another 20,000 were turned away when officials closed the doors to MSG about 4:30 p.m. — three hours before Trump went on.

“They could have sold the Garden twice,” the source said.

The source noted that Trump supporters just kept coming from all directions long after the last guests were admitted to MSG.

One supporter said he had been in the wrong line for two hours — and then was denied entry.

“I don’t mind, though,” he said. “We’ve been to Trump rallies before, so missing one is no big deal.”

Pushed away by law enforcement, some Trump supporters who didn’t make the cut to get in swarmed nearby bars and restaurants — where they changed the channel to tune into the rally.

At Avenida Bar across from the iconic venue, for instance, television sets were tuned into the rally and patrons were seen wearing MAGA hats, drinking, and chatting with one another.

“This definitely isn’t the usual crowd, they asked for all of the televisions to be turned onto the rally. I’ve already been called a Mexican once,” one employee at a nearby restaurant who declined to be named told The Post.

Those hoping to get into MSG faced stiff competition as some people lined up outside as early as Friday.

By early Sunday, roughly 10,000 people had assembled to get in, per an estimate by The Post corroborated by local cops.

Standing outside in the nippy weather, many MAGA-loving fans waiting in line had fought through urges to go to the bathroom and were peeved by the lack of nearby porta-potties, which prompted some to relieve themselves in bottles, which littered the pavement leading into the venue.

New York is highly unlikely to be competitive during the presidential election, but the Trump-Vance ticket is hoping to reap the benefits of headlines and media attention from his venture into deep blue territory during the campaign’s home stretch.

Additionally, the rally will likely double as a means of ginning up turnout for some down-ballot Republicans locked in tough races and chipping away at the popular vote.

Trump himself has been optimistic that he can pull off a political upset and “can take New York,” but most experts believe that is improbable.

The Queens-born former president has swung through the Empire State for campaign events before, including a May rally in the Bronx and a September rally in Long Island. He also stumped in California earlier this month.

Trump is not the only one heading into unfriendly territory. Vice President Kamala Harris campaigned this past Friday in Houston, Texas, a state that has long been a Republican stronghold.

Joining Trump at the rally was a roster of MAGA stars such as space and electric car guru Elon Musk, biotechnology entrepreneur Vivek Ramaswamy, GOP vice presidential hopeful JD Vance and more.