A normal person who possess a Job, usually knows or learns around 2-3 Skills at maximum in their lifetime. It is also possible for a person to never gain access to some of their Skills, because the right condition for them to bloom or be accessed haven't appeared before them. But on the other hand, if a person goes through arduous training, they maybe able to unlock more of their hidden Skills.
While its extremely rare to gain a new Skill; it is on the other hand easily possible to learn or gain a "Derivative-Skill", which as the name suggests are Skill which are derivative of the original Skill. These Derivative Skills could either be a power-upped version of the original Skill, or might be alternate type with a different function, be works similar to the original Skill. A Derivative-Skill can be gained from practice, increased experience or heightened emotions.
There are some Skills which are extremely unique in characteristics and have the ability to affect the over all abilities of a person. Like for example, the Skill "Mana Manipulation" allows a person to directly control their own mana, and allows them to either activate or use spells without the need of incantation or a magic circle.
There are also Skills which help increase the 'affinity' of certain aspects for a person, to allow them a easier access to use them, like if a person has an elemental 'affinity', that person can utilize a Spell of the said element with more efficiency, power and is able to cast them at a much faster rate.
The Ancient Magics are also considered Skills.