I think that a comic strip of more than six frames can be developed. In the end, the formation of this comic strip is going to be dependent on what you deem as important enough to include. What makes this task remarkably direct is that the work is six chapters, essentially, and you could construct two frames per chapter and get a 12 framed comic strip that addressed the essential points in the novel.
I would think that one frame could be devoted to the first chapter to the "brush" and setting that will serve as the backdrop for the climax of the novella. Perhaps, another frame could be devoted to the exchange between George and Lennie and reveal a small part of their own background. Another two frames could be devoted to their work on the ranch and the challenges with Curley and Curley's wife. The next two frames could include the integration of the Candy into their dream and the challenges they face in getting the money. I think that you would have spend a frame with the interaction between Curley's wife and Lennie. The mob being led by Curley to get Lennie and George faced with the brutal decision that confronts him at the final installment of the novella could constitute the closing frames of the comic strip.