Sorry, I didn't realize this has been in for days and languishing in the moderation queue. I moved it from the inner art submission forum to this one which is visible to you and everyone else - in case others have a similar question.
The answer to your question is as little as 1, and as many you want. As for images size/submission method, I noticed the current maximum size for image attachment was too low for the larger, higher resolution artwork, so I've bumped it up to just under 100K. However, my recommendation, if you have several images to send, is to zip (or rar) the images into one file, and attach the resulting file. The maximum size for zip and rar uploads is over 2Megs and should be able to accommodate hi-res artwork. If it's just a few images, you may attach it to the post directly, and the new max upload size should allow that. Finally, you may also submit links pointing to the images instead if that's more convenient for you.
Hope this clarifies everything so, keep on submitting, everyone!