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.

I cannot answer the question, but I immediately thought of 4-hour-work-week, because in that book the author describes a system involving 2 goals which I found quite handy.

You choose 2 goals, than you set a dreamline for each one. The first gets a dreamline 6 months from now, the other 12 months down the road. Now the nice thing is, 6 months later you can set another 12 months goal and will have again, one 6 month goal and one 12 month goal. That's nice because that way you have one project more in the beginning phase and another one in the finishing phase.