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.
One goal means you have no choice in what you are focusing on. At least with two goals, I can pick the better of the two to fit a specific mood or situation I am in (e.g. maker's versus manager's time per Paul's recent essay).
