How many dishes are there on page 1 of menu ID12882?
page 1 refers to page_number = 1;
SELECT SUM(CASE WHEN T1.page_number = 1 THEN 1 ELSE 0 END) FROM MenuPage AS T1 INNER JOIN MenuItem AS T2 ON T1.id = T2.menu_page_id WHERE T1.menu_id = 12882