There are 2 senators from each of the 50 states.
All senators are present, therefore there are 100 senators.
48 senators voted in favor of a treaty.
Let x = the number of additional senators who should vote in favor of the treaty to achieve 2/3 of 100.
That is,
48 + x ≥ (2/3)100
x ≥ 200/3 - 48
x ≥ 18.667
x should be an integer, therefore
x ≥ 19
Answer: 19 or greater