Due to the essence he put into the Ring, Sauron's existence was forever bound to the Ring, but that doesn't mean that Sauron is powerless without it.

When Sauron lost the Ring and the power within it, he was able to gradually regenerate that lost power, which was invested in the Ring, on his own, without needing to withdraw the power back from the Ring, while the power in the Ring remains in existence and undiminished.

While Sauron can regenerate the internal power be externalised and later lost, it doesn't indicate if any limits exist to that. We know he could no longer inhabit a fair form, but that may be working on a different mechanic that what we're discussing here (being that the fair form was in his possession from before his corruption into Morgoth's first lieutenant).

If the Ring had not been destroyed but remained out of Sauron's reach, how much power can he regenerate on his own? By the time of the LotR books/movies, he could inhabit a corporeal form again, though not yet able to fight like before.