すみません、毛布をもう一枚いただけますか？ — Excuse me, could I have one more blanket? (Ryokan · Accommodation, Formal — to staff, JLPT N4)

Gesture & etiquette

Mimic covering yourself with a blanket.

If the room is shared, ensure everyone in your group is okay with an extra blanket being brought in. Do not take blankets from other rooms or communal areas.