If you look at the data, it’s pretty interesting.
White, male, not college educated, older (over 45) is one large bucket. Interestingly, they tend to be solidly middle class. Trump got lower support with voters with income under $50,000 than he did with voters who make more than $50,000. Their big concerns are immigration and terrorism. Many fear a return of recession. If I were to pick a unifying theme for these voters, it would be fear. They also fear the erosion of their rights by the advancement of the rights of others. This group will say that Democrats have talked too much about identity politics, but what they mean is that they feel left behind, pushed aside, and as if they are losing while or because other people are gaining.
A smaller bucket would be higher income bracket pragmatics who are hoping to get fiscal benefits from a Republican administration.
In terms of numbers, there are 2 different ways to reach that. Trump seems to be holding at 90% of Republicans (24% of US voters identify as Republican). 24% * 250,000,000 US voters = 60,000,000
Second method is via his approval rating. 538 puts him at 42% approval with US adults (not voters, or likely voters, but adults). His disapproval rating is 52%.
If we take 250,000,000 estimated voters, and apply the 42%, his base is ~105,000,000.
I’m comfortable with the lower number, given his performance in the 2016 election. I wouldn’t be surprised if it’s lower.