Hard to say, and probably impossible to test.
At the B85M-P32's launch, 8GB was the highest (consumer) single module available. That's still true for unregistered DDR3.
So 2x slots and 8GB maximum available per = 16GB max support.
Theoretically though, there's no reason a 2x16GB kit couldn't work, but an unregistered 2x16GB DDR3 kit does not exist.... AFAIK anyway.
Realistically, I can't think of many justified reasons for >16GB on a B85M with, at best, an i7-4790.