I've always been attracted to the number 10 (hence my pseudonym). It's simple, logical and powerful. It's easily derived from human life--ten fingers and ten toes. It is the basis of logarithms. We measure time in decades, and 10 decades is a century. From ten we derive 100, 1000, 10,000, etc--all simple tools to understand quantities of various sizes. We even have 10 Commandments and 10 amendments to the Bill of Rights.
3 is a very convenient number for describing processes with steps--steps 1, 2 and 3. The beginning, the middle and the end. The past, the present and the future.
Elliott Wave Theory indicates that financial markets often move in 5 waves: Up, down, up, down and up. Each of the 3 up-waves move the price to a new high, and down-waves allow the market to correct. This 5-wave tendency may point to some deeper inherent tendency in large-scale human interaction or crowd behavior.
Even numbers like 2 or 10 are of course good if symmetry is what I'm looking for.