Basically, there is no limit on what kind of materials you want to make, or the quantity, as long as it's fine within the site rules. Since it's materials, you may want to use the word "material" in order to clarify that you are making them available for others.
Please ensure you heed the creator's disclaimer and depending on how they set their rules, there may be cases where you can't use them for a specific purpose.
It is also a sign of respect to set an image response, linking back to the creator's work if you've used them for your works during the submission, or credit them by stating where you got the materials, and so on.
※Even if they did state that it's unnecessary or not needed, you are still advised to do so as a sign of thank you.
Basically, as a form of etiquette (especially in Japan), you CANNOT use the materials for derivative distribution.
Commercially Allowed
Main Article : Commercially Allowed
Materials that can be used for commercial purposes, or sales.
Royalty Free
Main Article : Royalty Free
Materials that are only allowed to be used under certain circumstances.
Copyright Free
Main Article : Copyright Free
Materials that can be used in any shape or form.
