For me, two-thirds of 24 is clearly 16 votes. But this issue is more than just mathematics—it is about what the Constitution actually means by “two-thirds of all the Members of the Senate.”
If four senators are unable to participate, I don’t think we can automatically change 24 into 20 and reduce the requirement to 14. The legal basis for changing the denominator must come from the Constitution, Senate rules, or a proper legal interpretation.
So, my simple understanding is 16 votes, unless the competent constitutional authority determines otherwise. In matters this important, the law—not convenience—should guide the final decision.