。
The reservation number is 1003.
Literal
Reservation number [topic-は] 1003 is-[polite-です].
Minimal polite copula sentence giving a number. The kanji 予約 combines 予 ('in advance') and 約 ('promise, contract') — a transparent two-kanji compound learners can decompose. Numbers with 番号 ('number') often take は rather than が when reporting an identifier.