This is probably the short story "The Usual Santas" by Mick Herron, published in the collection The Usual Santas in 2017, and adapted for the Wall Street Journal as Santa #9: A Christmas Short Story, from which the quotes below are taken as I couldn't find a copy of the book.
Eight men work as Santas in the same shopping mall, called Whiteoaks:
The Santas turned up one by one. Whiteoaks had emptied of shoppers, but still: It would never do for two Santas to be seen together in public.
They realise there is an impostor among their number:
“Excuse me,” said Santa in a loud voice. “But I don’t have a glass!”
The Santas’ chatter died away.
“Well, someone must have two,” said Santa, jovially.
“Nobody’s got two,” Santa said. “That’s the point.”
“What’s the point?”
“There aren’t eight of us here,” Santa said. “There are nine.”
After trying for some time to figure out who is the ninth Santa, they start to realise that the "real Santa" (if such exists) might be needed there more than elsewhere:
The Santas nodded, one after the other. It was true. Whiteoaks was horrible, unless you liked vulgar commercialism writ huge, in which any suspicion of non-franchised individuality was stamped out before it made waves. The trouble was, the Santas had few alternatives as far as employment went. The local shops they’d once Santa-ed for had closed when Whiteoaks opened.
“But don’t you see?” Santa said. “That’s precisely why he’d come here!”
Santa said, “How do you mean?”
“The whole point of Santa is that he goes where he’s needed.”
“Like Whiteoaks? Ha!”
“I meant ho!”
“Exactly like Whiteoaks,” Santa insisted obstinately. “It wouldn’t know the meaning of Christmas if it came with a buy-one, get-one-free sticker. It’s crying out for Santa, for heaven’s sake!”
“But it has eight Santas,” Santa said. “It has us.”
A pleading note had crept into his voice.
“But it doesn’t have the real Santa,” Santa said quietly. “A Santa to teach it that profit isn’t everything.”
And the ending of the story:
What became known as the Great Whiteoaks Christmas Looting was never solved. Whoever coordinated the daredevil heist had somehow contrived to get hold of a master key, which not only gave access to every shop in the complex but also allowed every alarm and CCTV monitor to be switched off. Nor, given the tendency of store managers to estimate losses upwards for insurance purposes, was it clear exactly how much was stolen. Police investigations did suggest, however, that some very big sacks must have been used.
Nor was there any obvious connection between the daring robbery and the appearance, on Christmas morning, of some very big sacks on the doorsteps of children’s hospitals and foster homes. The sacks contained toys and games, and books and clothes, and sporting goods, and any number of mobile phones and PlayStation consoles, and some little sewing kits, and various beauty products, and some very expensive jewelry. This, the adults in charge of the various establishments swiftly liquidated into cash to ensure that all their charges’ future Christmases would be celebrated in an appropriately festive manner. And also to give themselves a small raise, because it was valuable and underappreciated work they did.
Back at Whiteoaks, the only thing approaching a clue that was ever discovered came to light some weeks later, when a truck arrived to collect a recycling bin. As it was moved, a large red and white bundle rolled into view. This turned out, on closer inspection, to be made up of nine Santa suits and nine Santa hats.
And eight false bushy white beards.
I spent some time trying various web search options for this, and ultimately the winning query was
"short story" "the real santa" mall santas "one of us".