Quick answer — The Merry Friends of Iwatobi performed 1 anime theme, including "What Wonderful Days!!" from Free! -Take Your Marks-.
The Merry Friends of Iwatobi performed the anime theme songs "What Wonderful Days!!" (Free! -Take Your Marks-).
The Merry Friends of Iwatobi's first listed anime theme is "What Wonderful Days!!", the ending of Free! -Take Your Marks-.