Voldemort probably cast a Refilling Spell on the potion so that it topped itself up automatically.
We can't be 100% sure how the potion refilled itself since there are some inconsistencies in how it's described. However, I think we can make an educated guess that Voldemort placed a Refilling Charm on the potion. As a result, the potion magically refilled itself every time the basin was emptied.
On one occasion, Voldemort seems to refill the basin manually. In other words, he takes the initiative to cast a spell to create more potion, suggesting that the basin didn't refill automatically.
“Kreacher drank, and as he drank, he saw terrible things...Kreacher’s insides burned...Kreacher cried for Master Regulus to save him, he cried for his Mistress Black, but the Dark Lord only laughed...He made Kreacher drink all the potion...He dropped a locket into the empty basin...He filled it with more potion."
“And then the Dark Lord sailed away, leaving Kreacher on the island. ...”
(Deathly Hallows, Chapter 10, Kreacher's Tale).
However, Kreacher didn't seem to refill the basin when he went back to the cave with Regulus. It seems from his account that doing so wasn't part of Regulus's instructions. It's doubtful whether he had the magic to duplicate potions anyway.
“And he ordered - Kreacher to leave - without him. And he told Kreacher - to go home - and never to tell my Mistress - what he had done - but to destroy - the first locket. And he drank - all the potion - and Kreacher swapped the lockets -
and watched...as Master Regulus...was dragged beneath the water...and...”
(Deathly Hallows, Chapter 10, Kreacher's Tale).
It's possible, though doubtful, that Kreacher did refill the basin but just didn't tell Harry that he did. Yet it's clear from the account of Dumbledore and Harry's time in the cave that neither of them refilled the basin. Yet when Voldemort revisited the cave the basin was full of potion.
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 -
(Deathly Hallows, Chapter 30, The Sacking of Severus Snape).
The best way of resolving this inconsistency is to say that Voldemort was applying the magic to the potion that made it refill automatically when he visited the cave with Kreacher. After all, he was still testing the defences of his Horcruxes at that time. Having seen the potency of the potion, he refilled the basin and applied a spell to make the potion top itself up whenever it was drunk.
I think we can be sure of the spell that he used since we see Harry use it earlier in the series.
The Felix Felicis gave Harry a little nudge at this point, and he noticed that the supply of drink that Slughorn had brought was running out fast. Harry had not yet managed to bring off the Refilling Charm without saying the incantation aloud, but the idea that he might not be able to do it tonight was laughable. Indeed, Harry grinned to himself as, unnoticed by either Hagrid or Slughorn (now swapping tales of the illegal trade in dragon eggs) he pointed his wand under the table at the emptying bottles and they immediately began to refill.
(Half-Blood Prince, Chapter 22, After the Burial).
It makes sense that Voldemort used a Refilling Spell on the potion when he visited the cave with Kreacher. Thereafter, the potion refilled itself magically.
