？。
Thirteen people? That's all?
Literal
13 people? Only that-much [emphatic-かよ].
A short, disappointed exclamation. そんだけ is the colloquial contraction of それだけ ('only that'). The sentence-final ~かよ combines か (question) with よ (assertive) to express disappointment or incredulous push-back. たった ('merely, only') intensifies the smallness.