Just an observation here....12 is a great number and you can go to 25 or 30 if people want to participate. BUT you have to increase the time frame for people to be able to achieve the work. So if, for instance, you have 25 people doing this, allow 3 months for people to complete their drawings without feeling rushed.
Everyone has a life to lead and can't mass produce cards or drawing fatigue will set in quickly, especially for those working as well as drawing.
Also the same people may not want to participate over and over in quick succession. That is another recipe for disinterest setting in. Expand the time frame, ensure that those participating MUST be prepared to follow through to the completion to avoid disappointment and remember to have fun!
When I started this initially, it was a one off. It was not intended to be an ongoing monthly trade. Now, of course, this can happen, but I do believe that you won't get the numbers of participants if ATC trades are done too frequently.
Just my two cents worth!