Not arbitrary per se. I like having two goals because then I can directly compare the goals when making a decision on what to try to accomplish. It becomes an either-or decision. If I am in the mood to complete something quickly, I can look at both goals and pick the one with the shorter time investment. I actively try to position goals against each other so that they help me stay balanced (e.g. one goal is fitness related, one goal is work related). Having 3 goals tends to introduce too many points of comparison for me.