There is no mention of mana in Harry Potter.
It is meant to be very different than an RPG or any sort of material that you are used to.
Magic in the Potter Universe, biologically is a genetic ability to practice witchcraft.
One of the main things that changes a user's ability to use magic is their emotional state... which affects the ability to use magic effectively one such example is Nymphadora Tonks..
an excerpt from Harry Potter Wikia on magic
http://harrypotter.wikia.com/wiki/Magic
For example, Nymphadora Tonks temporarily lost her power as a
Metamorphmagus after suffering severe emotional turmoil and sadness
over her grief for the death of Sirius Black, and when Remus Lupin
would not return her affections. In effect, the form of her Patronus
changed to reflect her love for him. The form of her Patronus changed
to reflect her depression. In 1995, when Mad-Eye called her by her
first name, her hair temporarily turned red. Wizardkind are also
weakened when in the presence of Dementors for prolonged periods, as
said creatures attack their prey psychologically by making them recall
their worst memories, leaving the victims physically vulnerable.
As related to Harry by Dumbledore during their private lessons, Merope
Gaunt only demonstrated any magical ability when removed from her
father's oppression, but then seemed to lose it again when her husband
abandoned her."
So far I have not seen any instance that magic in Harry Potter has anything to do with a reserve pool of magic that a person can use, most times magic is done through the use of a wand as well.
Even in such instances where magic was done without one there is no decrease in power.
Also according to the Harry Potter Wiki there are only a few limits to magic power.
Rule of Conjuration: While it is possible to conjure things out of
thin air, it is far more tricky to create something that fits an exact
specification rather than a general one; moreover, any objects so
conjured tend not to last.
Rule Against Resurrection: It is also impossible to resurrect the
dead. While corpses can be transformed into obedient Inferi on a
living wizard's command, they are little more than zombies with no
soul or will of their own. It is also possible via the rare Priori
Incantatem effect to converse with ghost-like "shadows" of magically
murdered people. The Resurrection Stone also allows one to talk to the
dead, but those brought back by the Stone are not corporeal, nor do
they wish to be disturbed from their peaceful rest. The result of such
a summoning is usually detrimental to the summoner.
Rule Against Immortality: Likewise, it is impossible to make oneself
immortal unless one makes use of a mystical object of great power to
sustain life (such as the Philosopher's Stone created by Nicolas
Flamel or a Horcrux, the latter having been used by Lord Voldemort and
Herpo the Foul). Becoming a ghost is also an option for wizards and
witches; however, it is said that this is "a pale imitation of life".
Whether or not ghosts are truly sentient beings possessed of an
independent existence is unclear; Snape states that a ghost is merely
"the imprint of a departed soul left upon the earth".