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In Harry Potter, wizards can use any magic they know at any time, regardless of how many wizards there are, how tired they are, how long they've been casting spells or how much magic is used around them.
- We have huge battles like the Battle of Hogwarts in DH, where hundreds of magic users are concentrated in a few square km and use massive amounts of spells.
- We have battles that last hours and where the combatants are doing lots of running, casting and talking, like the Battle in the Ministry of Magic in OotP.
- Magical abilities also don't seem to be negatively affected by general health or age. Dumbledore, who is well over 100 years old, is able to duel Voldemort to a standstill.
- Harry could keep up the Priori Incantatem spell in GoF for a long time, even after doing the 3rd Triwizard Cup challenge, taking a Portkey over a long distance, being tortured by Voldemort and all the other stress accompanied by that.
When I look at other fictional works involving magic, they all seem to have a limit of sorts. In The Dresden Files, magic requires preparation, both in making sure you have enough of it and having certain material parts ready, like protective circles, special reagents or even jewelry to act as a focus for the magic, and using magic tires you out. In many versions of D&D, casters are limited by how many spells they prepared. A huge number of videogames use mana to limit casters. Just about any fantasy series I've consumed involving magic has limits on how much magic can be used by a single person, either by making the user get tired from using too much, by making the amount of magic in a certain location limited, or by coupling it to preparations or emotions.
Has J.K. Rowling or anyone else who worked on Harry Potter related content stated whether there are any physical limits to how much magic a single Wizard can use in a certain timespan?