As many as you want during the challenges, as long as YOU created them! 🙂 You can submit some even after challenges and we will feature them on the site.
It depends on the contest. For some, the winner will be chosen randomly. For others, it will be based on feedback from the community, while others might be selected based on quality & skill. The challenge will indicate how the winners will be chosen.
It can be jpg, png, svg, ai, eps, gif, or any other image format. No zip files.
This can include photos of physical drawings that you want to submit, they don’t have to be digital.