Here are my guesses. Justices Breyer and Ginsburg are pretty obvious votes for the mandate, as they dissented in United States v. Lopez. Justices Kagan and Sotomayor seem like safe votes for the mandate, even if only for the reason that there is almost no opposition to the constitutionality of the mandate in the Democratic establishment from which they were appointed. Chief Justice Roberts will likely vote to uphold the mandate given the very expansive views of the Necessary and Proper clause that he signed on to just recently in United States v. Comstock. I suspect Justice Kennedy will vote to uphold the mandate given his concurring opinion in United States v. Lopez. And I’m pretty sure Justice Thomas will vote to strike down the mandate given his views of the Commerce Clause. In contrast, I don’t have good sense of where Justices Scalia and Alito might come out.
Putting the numbers together, I expect 6 votes for the mandate, 1 against, and 2 uncertain. If my numbers are right, the mandate will be upheld by a vote of anywhere from 6-3 to 8-1.