We don’t know. (but there’s a guesstimate near the end of this answer)
Barring some kind of magical secret sauce, human brains work on the basis of physics. And physics can be simulated by a computer to any precision we desire.
Which means that in principle, it’d be possible to construct a human brain inside a computer by letting the computer simulate the physics that makes a brain work. Problem is, we don’t have a good enough understanding of how the human brain works to be able to tell exactly how much of the physics we need to model to which accuracy. It’s also very likely that an AI could be