No real upper limit (outside of absurd numbers I suppose) on how many characters you can submit. Either via a group image or individuals. Be mindful that group images might be harder to place unless they are rather tightly grouped.
Safe only. OCs only. Community OCs, personal OCs, or OCs you get permission to draw are all fine.
Poses should be fairly standard with winter themed accessories if you want to have a character wear anything. Flying poses should be plenty available as well. Accessories themed after official characters are fine (plush figures/dolls, pictures on shirts, badges, buttons, etc.) while the official characters themselves aren’t.
Feel free to ask if you have anything specific you want answered. Otherwise most standard collab rules should apply overall. I usually check in at least a couple of times a day to look for updates. It’s quite late for me right now so I was about to head to bed though so I will answer what I can when I see it.