My suggestion would be to take leather as many ply as you will need to sew. Then add at least one more and test the machine. The reason for more is to be quite sure you will not be using it at the limit of what it can do. Even try to see how much more it can do if pushing the limit. For example if it will handle 2 more ply then you think you will be sewing that's good. I do not know these machines, what I advise would apply to any machine.