Answer
$19$ senators to $52$ senators
Work Step by Step
With $2$ senators from each of the $50$ states, the total number of senators is
$$
2\times50=100\text{ senators}
.$$Since an approval needs the vote of two thirds of all senators, then the number of senators needed to approve a treaty is
$$
\frac{2}{3}\times100\approx67\text{ senators}
.$$Since $48$ senators have already voted for the approval of the treaty, then the least number of additional senators needed to approve the treaty is
$$
67-48=19\text{ senators}
.$$Hence, an additional $19$ senators from the remaining $52$ senators are needed to approve the treaty.
