You can't 'grind' Ragnarok. You can only run it once per day. So that's 7 per advent, 8 if you log in early enough before daily reset.

And if it means anything, I managed to clear Ragnarok with my 32k wind team with 2 Quetzalcoatl, Mordred and no SSR himes. Very little assault% too, not a single weapon higher than Lv6 assault. Might have gotten lucky.