Digimon's most insane use of this trope, however, is the climax of Digimon Next, where literally every single character the kids and their Digimon have met who isn't dead (and even a few who are) show up to help against the Final Boss, ranging from The Mentor to background characters to even the surviving villains. Unfortunately for them, said final boss verges on The Omnipotent, so he simply obliterates them all in one shot and the entire dimension while he's at it, leaving only the main characters alive. He's ultimately beaten simply be being convinced by the protagonists that he's wrong, and he restores everything and everyone he destroyed.
The Anime Adaption of Black Cat pulls this trope in the final few episodes. When Eve is kidnapped by Mason in order to activate the Eden Project, Train calls on literally every other surviving character (More than fifty) to aid in the rescue. Whilst the original call extends out to the remaining Chronos Numbers and the Sweepers Alliance, word spreads and Train and company eventually receive assistance from the likes of the rogue Apostles of the Stars, the kids from Leon and Tim's hideout, and later even Creed and Echidna.
The final Best Student Council arc, where nearly every minor character who showed up once and got named returned to help the titular student council.
Mermaid Melody Pichi Pichi Pitch always brought in the secondary trio, Kaito, and that season's orange princess in the end, even though throughout the series, they were either useless, unawakened, incapacitated or unwilling to do the job.
Sort of done in the climactic battle with Kid Buu in Dragon Ball Z: Goku continuously asks the people of Earth to lend some of their energy to his Spirit Bomb, but none of them want to because they can't see Goku, don't know who he is, and the few people who do lend their energy end up really exhausted afterwards. It takes Hercule/Mr. Satan, the Fake Ultimate Hero of the DBZ world, to convince the population to lend their strength.
In the anime version, there are some people who answer Goku's request, and we get to see some faces we haven't seen since Dragon Ball (Bora and Upa and the group from Penguin Village come to mind). We also get to see what's become of Lunch and Seventeen.
While not exactly calling for direct aid, Eyeshield 21 has Every ace from every important team in Kantou show up to help the Devil Bats train for the Christmas Bowl finals.
When it comes down to the final battle in Violinist of Hameln.(pictured◊), a truly ridiculous amount of reinforcements show up (including just about every minor character capable of taking to the field).
In Naruto, the Leaf Village gets the Hidden Sand Village to help them bring Sasuke back when he leaves Konoha for Orochimaru. Later, in Shippuden, its the Sand Village's turn to call for aid when they need to rescue the Kazekage.
The Raikage does this by issuing calls for an immediate summit of the Kages to discuss the threat of Akatsuki. With the threat posed, he insists that all the vilages must unite. He had previously ignored these very warnings from Konoha, and only acted when his own brother was lost.
The final battle of Vandread brings back every character and civilization that ever displayed the ability to pilot a spaceship.
In Durarara!!, Mikado calls on everyone online at the Dollars website to help rescue Anri from the Yellow Scarves
The last episode of the hentai title Meiking, every group Cain helped (or spared) along the way showed up to assist in the final showdown with Cain's evil rival, Francis.
In Tengen Toppa Gurren Lagann episode seven, a number of characters (such as Kittan and Dayakka, amongst a bunch of newly introduced folks like the twins) arrive to help the Gurren-Dan capture what will become the Dai-Gurren. They stay around afterwards, however, and permanently expand the Gurren-Dan.
Towards the ending of Mai-Otome, BOTH sides in the final battle call upon reinforcements in the form of Otome from other nations, all seen briefly way back in episode three. And once the final battle breaks out, the teachers and entire student body of Garderobe comes to help the heroes.
Happens in the manga too. When the real Princess Mashiro resurrects herself and dark versions of the Himes from the prequel, the protagonist(s) get help from Garderobe, the other nations, and even Shwartz in fending them off so Manshiro can get to the castle for the Final Battle.
In the OVA series Golden Boy's final episode, Kintaro Oe calls upon each of the women whose hearts he'd won in previous episodes for the finale in which he works for an anime production company (they each had talents to offer).
In the final episodes of G Gundam, all the Gundams of the world unite to help the Shuffle Alliance defend Earth. Naturally, they include nearly all the opponents the main cast has fought up to this point (and creating a Mythology Gag since several others are based upon past series Gundams; including an early appearance of Wing Gundam), all putting aside their differences to defend the world they love.
Near the climax of the G/S/C Story Arc of Pokémon Adventures, things were looking grim for the Pokédex Holders as they're fighting a losing battle against both Ho-oh and Lugia. Just then, they noticed something in the distance. Bill had finally fixed the Pokémon Transporter that the new Team Rocket had sabotaged, and with the director of Goldenrod Radio spreading the news, every single minor character introduced in the arc (and even some from the RGBY arc, meaning that it's likely every trainer in the Johto and Kanto regions) had sent their Pokémon to help the heroes, producing a huge-ass army that rushed the two Legendaries and quickly tamed them.
In the finale of the XYZ arc, all of the Gym Leaders team up with Ash and his friends together to stop the Zygarde rock monster.
Ties into Write Back to the Future in Rave Master when Seig Hart, in the past, writes letters to all of Haru's friends so that they arrive to supply much needed support in the final battle. How he knew when the battle would occur...
Happens during the climax of Sailor Moon Super S. The Senshi need to use the Golden Crystal to defeat Nehelania, but when Sailor Moon tries to do so, she fails to activate it's power. Nehelania reveals that the power source of the Crystal is the beautiful dreams of humanity...but mankind has lost the power to have those dreams, depriving the crystal of it's power. Helios insists this isn't true, prompting Chibi-Moon to use the Crystal to call on all the people of the world with beautiful dreams to lend the power of those dreams to them. And, in a montage showing almost every character previously targeted for having beautiful dreams, they do just that, breaking Nehelania's spell over the Earth and repowering the Crystal.
In Fullmetal Alchemist, when Ed and Al call on the help of all their friends from places they've been (like Briggs) and from friends (like Izumi) to help them defeat the Homunculi and Father in the Final Battle.
Transformers Cybertron in the final battle against Galvatron, the Autobots are joined by the forces of Velocitron, Jungle Planet, Gigantion and the Decepticons under Lugnutz leadership.
Raynesia does this in episode 18 of Log Horizon. She offers to go to Akihabara to ask the adventurers directly for help in defeating the goblin armies after talks between the nobles and adventurer leaders goes nowhere. Although she doesn't think anyone would seriously help, they're all smitten by her speech and looks, as well as being hyped up for a major event/quest in the game. As a result, practically everyone present volunteers.
Shimoneta provides a Well-Intentioned Extremist example. For the first eight episodes, SOX manages to foil the Public Morals Committee and the student council at Tokioka Academy. Anna finally accepts that she can't stop them on her own, so she tearfully calls her father, Matsukage, for help. The scene shifts to his office as he assures her he's sending one of his best. The camera pans left, revealing Oboro standing behind him.
Kemono Friends: The Lucky Beasts call in most of the Friends to help battle the black Cerulean in episode 12.