Aster focused his attention in the place where he created the first fraction of the cycle, which Esmeralda taught him, the whole body of a cultivator had meridians, since spirit energy had to flow through every place.
They aren't like veins whose number is always the same in the body, but their number varies from person to person, not to mention new ones can be created, depending on the manual used to cultivate or other factors.
For example, Aster's movement technique, Lightning Step, made him create meridians in his legs which other people don't have, he also had to learn the way to make spirit energy flow into them to activate the technique, that's what manuals were used for mainly.
Also it's like one can simply just learn techniques to create more and more meridians, thinking it would increase one's capacity to circulate spirit energy, there is a limit as to how much one's body can endure.
