暴動が起こったとき、当直の監視員はたった１人だった。

、。

When the riot broke out, there was only one guard on duty.

Literal

Riot [subject-が] occurred when, on-duty [possessive-の] guard [topic-は] merely one-person was.

とき introduces a temporal clause ('when the riot occurred'). 当直の監視員 chains two sino-Japanese compounds: 当直 (on duty) + 監視員 (guard). たった + counter is an emphatic 'only/merely' — stronger than just 一人. The は on 監視員 sets up the topic to deliver the surprising fact of how few there were.