I originally noted this in my "Society6 Set Up and Promotion Strategy" - article, but I thought to write a small one just on this. I'm sure that I haven't been the first wondering this. In February 2014 when I discovered this, the limit was approximately 20 images within 2 hours. If you would try to post more in one session Society6 would just give you an error "This page is temporarily unavailable. Please try again later."
I'm not sure why they haven't described this in their pages, I can only guess. Maybe because they want to keep modifying this and having it written somewhere would just be too much maintenance. Anyhow, artists uploading more than 20 images at one go should be rare, because not too many artist have that daily output in their production. Exceptions are like me, who wanted to upload the whole inventory at once there.
Alright so just be aware of this. If you hit message take a break and continue some 2 hours later. It should reset after a while. If the upload limits has changed, please comment this blog so we'll get the latest news on this! =)
Thanks for reading, I hope this has been a help to you.