Like how stat points are gained. You start with 100 and 5 every even level and an additional 5 points for every 10 levels. So on level 20 since it's an even level and 10 you get 10 points.
Also another thing, when Maple eats a creature it's treated as the HP Drain attack being used on it. So she isn't gain any skills by eating, because a skill/ability that would Drain HP would have given her the same abilities.
Another thing, Maple's special equipment has skill slots in them. She gains a new skill slot for every five levels she obtains in her equipment. Her Equipment allows her to use a skill 5 times a day without expending any MP. This is why she can use Hydra which does use MP without actually having any. Another note, Devour also converts what it devours into MP, so she can use to gain extra MP which allows her to use her skills more often.
Last note, even though there are Classes, it has more to do with your choice of weapons. Most skills can be learned by any class or player. Skill limitations are for weapon sets, so the Skill exclusive Maple received is for characters wielding a Great Shield, not for the Great Shield Class, per say.
Further, if a skill doesn't suit you, you can trash it. Sally actually received the Giant Killer Skill, Maple had, but dumped it immediately since it was a passive skill. Sally's skill changing on her would mess up her ability to control and focus her skills and attacks, so that skill was actually a disadvantage for her.