Honestly, I don't know. I even thought about rating criteria based on count of assets but understood that it would be bad. Just Imagine some beginners they can make quality assets but it will take much more time than for professionals. Professionals have already know how to draw something quickly they have intuition and they have already overcome many difficulties. Many of those difficulties you need to overcome 1 time then it won't be difficult for you but first time overcoming concrete difficult is just like spending several hours or even days...

I am not sure but it seems to me it would be unfair to make such rating criteria but at the same time we can't compare asset with 20 characters and one unfinished tileset.

About minimal number I don't know but try to make something at least finished in the sense of MVP. For character it could be one animation (idle). For plat former tileset it could be basic 4x4 square of tiles...

It is first try of this jam and we have participants so I hope in the process we will find compromise rule that can allow to rate and compare but don't forget about "X Factor (overall enjoyment)". It is absolutely up to you because it shows how much do you like the pack. The pack can be high quality maybe you even rated all other criterias with 5 stars but you don't like at all so you can rate 0 stars this criteria. It is chance to avoid unfairness... ¯\_(ツ)_/¯

P.S Each Jam has community page where you can ask questions related to the jam. This question could/should be there ;)