Shulchan Arukh, Orach Chayim 283/שולחן ערוך·אורח חיים רפ״ג

11. We do not take a second Torah out on Shabbat to read the maftir because it only has two pesukim [note: this means that the passage describing the Mussaf offering of Shabbos contains only two verses, and no fewer than three verses may be read from a Torah scroll]