It will be determinable by how far apart atoms of carbon or boron can be before they are no longer considered "connected". Being only one atom thick, graphene and borophene will actually have less folding than a much thicker material because the folded edges will require the atoms to be farther and farther apart as the number of folds increases. I suspect by the second fold, the thickness of the layers in the middle will force the atoms at the folded edge apart and then you no longer have a single sheet of your material.it should be a matter (pun intended) of calculating the bend radius and comparing it to the distance between atoms to be considered still one continuous sheet. Now that I say it, I think one fold will be the max as two will definitely exceed the connection distance. So yeah, one fold.