Why 9 instead of 5 or 7 or 11 or 13?

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Why are there 9 justices on the Supreme Court of the United States (SCOTUS)? It is because there were 9 federal court circuits at the time. The time was 1869. 9 was settled on because the US had 9 federal court circuits then due to expansion into the west. Before that, the Court did not have a set number. At its low, 5 justices sat on the bench. At its peak, 10. It's not magic or even a very imaginative reason for there to be 9. We now have 13 federal court circuits, so an argument could be made that, based on the reason given for seating 9 in 1869, we should have 13 justices. Also, instead of lifetime appointments, should terms for the justices be reduced to a reasonable number of years, say 7 years with no reappointments? Remember, when the idea of lifetime appointments was written, few people lived past 40. What do you think? I SCOTUS too big or too small. Is a lifetime appointment too long considering the increase in human life expectancy since the Constitution was written?