40. You need 66 to ratify a treaty, but you can also pass a 'treaty implementation bill' which is basically normal legislation mirroring the language of the treaty, and you only need a majority in the house, and either a majority or a filibuster-proof majority in the senate. That's how NAFTA was passed.
Obama could have tried that with the Paris climate deal, but now it would require Trump's signature, or enough votes to override a veto (again including 67 Senators).
But both sides seem to be playing by a new rulebook here, in which the President makes international agreements without consulting Congress.
