From the moment Voldemort found out that his Horcruxes were being hunted until the moment he died was only a few hours. He found out about the hunt after the Gringotts break-in:
The scream of rage, of denial left him as if it were a stranger's. He was crazed, frenzied, it could not be true, it was impossible, nobody had known. How was it possible that the boy could have discovered his secret?
He immediately decided to check on and secure his existing Horcruxes:
But to be sure, to be utterly sure, he must return to each of his hiding places, he must redouble protection around each of his Horcruxes... A job, like the quest for the Elder Wand, that he must undertake alone...
While Harry was sneaking into Hogwarts, Voldemort was visiting the Gaunt shack:
Before any of them could parry the question with one of their own, Harry felt a terrible, scorching pain in the lightning scar. As he turned his back hastily on the curious and delighted faces, the Room of Requirement vanished, and he was standing inside a ruined stone shack, and the rotting floorboards were ripped apart at his feet, a disinterred golden box lay open and empty beside the hole, and Voldemort’s scream of fury vibrated inside his head.
When Voldemort was notified that Harry had been apprehended at Hogwarts, he was entering the cave hiding place:
The moment her finger touched the Mark, Harry's scar burned savagely, the starry room vanished from sight, and he was standing upon an outcrop of rock beneath a cliff, and the sea was washing around him and there was a triumph in his heart – They have the boy.
Voldemort chose to continue through to that location before heading to Harry:
Harry closed his eyes, and as his scar throbbed he chose to sink again into Voldemort's mind.... He was moving along the tunnel into the first cave.... He had chosen to make sure of the locket before coming...but that would not take him long....
Harry tells McGonagall that Voldemort is on the way:
“Yeah, I do,” Harry assured her. Somehow her panic steadied him. “Professor McGonagall, Voldemort's on the way.”
While he was still at the cave location:
In a distant part of Harry's brain, that part connected to the angry, burning scar, he could see Voldemort sailing fast over the dark lake in the ghostly green boat.... He had nearly reached the island where the stone basin stood....
When Voldemort finds out that the locket is gone as well, Harry says that he's even nearer:
As she said it, a wrath that was like physical pain blazed through Harry, setting his scar on fire, and for a second he looked down upon a basin whose potion had turned clear, and saw that no golden locket lay safe beneath the surface –.
“Time's running out, Voldemort's getting nearer, Professor, I'm acting on Dumbledore's orders, I must find what he wanted me to find!
By the time the people in Hogwarts had made arrangements to evacuate the students and fight, Voldemort had already arrived:
But then his scar seared and the Room of Requirement vanished. He was looking through the high wrought-iron gates with winged boats on pillars at either side, looking through the dark grounds toward the castle, which was ablaze with lights. Nagini lay draped over his shoulders. He was possessed of that cold, cruel sense of purpose that preceded murder.
Thus, there was no downtime between Voldemort finding out that Harry was hunting Horcruxes and the battle at Hogwarts. Once the battle began, Voldemort was focused on the battle, and on mastering the Elder Wand. He still believed that Harry did not pose a real threat to him in a duel. In his mind, Harry was only still alive due to a combination of luck, help, and technical wand issues, and Voldemort thought that killing Snape would resolve the last of those issues:
"One of us?" jeered Voldemort, and his whole body was taut and his red eyes stared, a snake that was about to strike. "You think it will be you, do you, the boy who has survived by accident, and because Dumbledore was pulling the strings?"
"Accidents!" screamed Voldemort, but still he did not strike, and the watching crowd was frozen as if Petrified, and of the hundreds in the Hall, nobody seemed to breathe but they two. "Accident and chance and the fact that you crouched and sniveled behind the skirts of greater men and women, and permitted me to kill them for you!"
"If it is not love that will save you this time," said Voldemort, "you must believe that you have magic that I do not, or else a weapon more powerful than mine?"
"You think you know more magic than I do?" he said. "Than I, than Lord Voldemort, who has performed magic that Dumbledore himself never dreamed of?"
Moreover, throughout this entire time Voldemort still didn't know that Harry had found the last Horcrux. In fact he considered it an impossibility that Harry could have found it:
As for the school: He alone knew where in Hogwarts he had stowed the Horcrux, because he alone had plumbed the deepest secrets of that place...
Generations of students had failed to find the diadem; that suggested that it was not in Ravenclaw Tower – but if not there, where? What hiding place had Tom Riddle discovered inside Hogwarts Castle, that he believed would remain secret forever?
Tom Riddle, who confided in no one and operated alone, might have been arrogant enough to assume that he, and only he, had penetrated the deepest mysteries of Hogwarts Castle.
Additionally, up until the last part of the fighting Nagini was still alive. She was only killed by Neville right before Voldemort was forced into combat:
With a single stroke Neville sliced off the great snake's head, which spun high into the air, gleaming in the light flooding from the entrance hall, and Voldemort's mouth was open in a scream of fury that nobody could hear, and the snake's body thudded to the ground at his feet –
He was searching for Voldemort and saw him across the room, firing spells from his wand as he backed into the Great Hall, still screaming instructions to his followers as he sent curses flying left and right;
Thus, up until the very last moments Voldemort still believed that he had two remaining Horcruxes. That being the case, even though he knew that Harry had been searching for Horcruxes, there was no pressing need to create new ones. By the time he realized that he was Horcruxless he was already mired in the final combat, and presumably could not have created a new Horcrux even had he wanted to.