There is a simple answer to this question: the name most go by is their second one.
Allow me to indulge in a bit of elaboration. Generally a child is first named by those who raise them. This initial moniker might be an aspiration on behalf of their parents, a reminder of some sort of truth or responsibility, or just something simple and easy to remember. As a child matures, they may decide this first name suits them well enough and carry it with them until the day of their passing. This is considered to be an auspicious portent indeed, as those who first named the individual can take pride in in the strength of their choice.
However, it is not uncommon for one to reach a point (generally when they come of age, but sometimes before that time or after) where they feel this first name no longer suits them. A child named for skill in archery may instead take to scholarly pursuits as an adult. One granted an ambitious title may discover a simpler one they prefer. Some who might have been given a name in hopes they would be loved may wish to be dreaded instead. Regardless of the reason, the individual will announce this new name to those around them and go by it from then on, with this decision respected.
Naturally, this too is considered to be good fortune, as those who helped raise the person taking a new name can assume their guidance brought the individual to the truth.
Mortal folk are not the only ones who choose new names, of course. Elves are notorious for the amount of monikers they take on in their impossibly long lifespans. Even more stately immortals will occasionally shift in identity on occasion. The most famous of these is Night Haven's Duke of Red Roses, whose identity once involved many flowers until he decided to embrace his favorite one alone.
As a final note, while the general sound and theme of one's first name (and often, any later ones they might have) is generally dictated by their species or the species of those who named them, region and the preferences in languages thereof also play a part, which is another reason for the variety of unique and expressive names in Outer World.