Sure, but that # is almost surely smaller than in 2016... Of course he still has a core base of supporters, but there were also plenty of fence-sitters last time who opted to roll the dice w/ the New Guy cuz they at least knew for sure they didn't like HRC. Whatever % of those voters out there who have soured on Trump and/or don't have nearly the same negative response to Biden that they did for HRC may not be huge, but some of them will probably sit this one out while everyone who actively switches sides essentially counts double since they both subtract votes from Trump while also adding to his opposition. Meanwhile as others pointed out, there's a lot more motivated anti-Trump solidarity on the Dems side than there was last time among the Bernie faction, so their turnout would be expected to increase (which is where the GOP's blatant suppression tactics come into play).

Not to mention that first group of 'swing' voters isn't just independents or loosely-affiliated w/ either party who may be persuaded to flip back & forth across various election/economic cycles, but this time even includes many lifelong solid Republicans (e.g., Lincoln Project) who recognize that Trump's personal behavior and disregard for process & institutions are big enough threats to the republic and our system of governance to warrant defecting to a reasonable & decent Dem who they simply disagree with on policy. He needed everything to break just right to capture all the swing states in 2016, essentially winning the election by only about 70K votes in key areas despite Clinton gaining ~3M more votes overall.

Where has he done anything to add to the base of people that supported him last time? I'd be willing to bet that he gets fewer total votes than before, while Biden gets a higher % than HRC did (less sure about total votes, depending on how difficult it gets w/ all the added obstacles to either in-person or mail-in ballots this year). The only question is in the geographic distribution, whether Trump can pull the upset again by repeating the same pattern of electoral sleight; obviously it doesn't do Biden any good to simply pummel Trump even worse in the states HRC already won.