Wikipedia contains expansive entries for forty named characters in Exo Squad, nine of them in The Squad alone. And they are not being particularly thorough at that...
Due to its long run, the number of characters in The Simpsons that could be considered regular is at least 30-50, possibly even more.
There is a poster that actually lists the characters that have had more than one episode, and it was well over 100. There are even T-shirts◊ of this (though this one is almost 10 years old, and enough characters have been introduced since then to fill another t-shirt size crowd shot.)
The Animals of Farthing Wood has a long list of characters at the beginning of the show. Many more get introduced as the series progresses, but only a handful of the original cast survives the series. (everybody else has died due to cold, old age, hunters, producers who just cut them out of the show without a single explanation, etc.) The survivors are Fox, Weasel, the Rabbits, Owl, the Squirrels, and Toad.
Justice League started with a core cast of seven heroes, and a handful of recurring villains. Starting with the third season, they make it a true league, and at one point state that they have at least sixty members (not counting a few rogue agents like Huntress and Hawkman), plus the villains, plus two of the Cadmus folk. This meant that at the very least, these characters would be seen in the background in the Watchtower, or in fights, and several got moments to shine.
Super Friends also started with just a core cast, but continued to add more heroes (and villians) each season.
The aptly named Legion of Super Heroes is the same idea applied to the DCU as seen in the 31st century. Its format, however, tends to be "the core plus one or two guest members," with larger numbers turning out for major threats.
Teen Titans' final season has the team meeting up with other young heroes, typically having one main Titan defending one or more less experienced members from villains. It's a good thing, too, since nearly every villain in the show's history, down to even long-unseen one-shot villains (but excluding most Big Bads) had also teamed up. It all comes together in a two-episode Final Battle. Even being dead when last seen was no excuse for a hero or a villain to not be in on that one.
Seasons 1 through 4 though don't particularly follow this trope: there's 5-8 major cast members and maybe a dozen recurrers.
The Spectacular Spider-Man has the obvious ones, Spidey, Gwen, MJ, Aunt May, Norman Osborn, Harry Osborn, JJJ... and then there's three cops, four co-workers at the Daily Bugle, and eight or nine other people at Peter's school, and that's not counting the seventeen supervillains and at least five future supervillains.
Aside from characters from the aforementioned series, pick out any popular Marvel character and it's a good chance they're on The Super Hero Squad Show.
Futurama, to the point where one of the last shots of the final DVD Movie was a massive crowd shot featuring every character except the children (to keep continuity with a line that stated they weren't there) purely as a fan-pleaser.
In one of the DVD commentaries, someone wonders what it would be like to have a machine that can mimic every character's voice, to which someone else replied "We do, it's called Billy West."
Gargoyles helped keep track of additional characters through a Cast Herd; everyone is connected through their primary associations. Of the main characters there is Goliath, Hudson, Brooklyn, Broadway, Lexington, Bronx, and Elisa; with Xanatos and Fox being second only to them. Then there are individuals like Matt Bluestone, Macbeth, and Demona. After that there are the various groups like The Pack, the Mutates, the Hunters, the Avalon clan, and the magical creatures like Puck, Oberon, and Titania. And then there are the characters that are the lesser seen including the England clan and family members that show up like Elisa's family, Foxs' father, and Xanatos' dad.
Total Drama Island, in a way; there are really only twenty-two contestants (upgraded to twenty-four as of Season Three), plus Chris and Chef, in the main cast with a few very minor cameo characters. But at least at the beginning of the series, any one of those twenty-two could have been considered a main character; there was no way to tell who was going to be "the star," and the focus changes between seasons (to some degree). This was quickly remedied, though, since a character is usually kicked off every episode. Every contestant winds up being an Ensemble Dark Horse with a chance for future Character Development, though.
The fourth season, Total Drama: Revenge Of The Island added thirteen more contestants, bringing the total count up to forty main characters.
The sixth season, Total Drama: Pahkitew Island, will add an additional 14, bringing that up to fifty-four.
The Venture Bros. has Team Venture itself, the Monarch and Dr. Girlfriend, the Monarch henchmen, the Guild of Calamitous Intent, OSI, the Order of the Triad, the original Team Venture, and dozens of other supporting characters.
Handy Manny focuses on quite a lot of the residents of Sheet Rock Hills. In fact, there are ten main characters: Manny, Kelly, and the tools.
Star Wars: The Clone Wars doesn't really have a regular cast aside from the obvious Obi-Wan Anakin and Ahsoka trio with Captain Rex, Artoo, Padmé often, and 3PO, and Cody occasionally thrown into the mix. Then we still have fourteen jedi, at least ten republic military officers (both clones and non-clones), six-six Republic and Separatist officers, six bounty hunters, and seventeen non-aligned characters, that have imporant roles in more than one episode. There's also a huge horde of about fifty one- and two-shot characters.
The Closing song of Phineas and Ferb "Roller Coaster the Musical" features almost every character in the first and second seasons from the important characters like Phineas, Ferb, Isabella, and Candace to the characters who are only seen in the background..
Adventure Time is quickly becoming this trope. The majority of the cast includes one episode appearances like Penny and background characters like Lollipop Girl and Gumdrop Lass. The show also had the distinction of nearly doubling the entire cast in just one episode. That episode was "Fionna and Cake", in which the main cast and Candy Kingdom residents were all genderbent.
The Fairly OddParents. Seriously, how can kids keep track of all those characters!? Just check out its character page on That Other Wiki. It's even got a subcategory for inanimate objects. Phillip the nickel, that rock in one episode that was more popular than Timmy...
Courage the Cowardly Dog provides a new character for pretty much every episode, all of which actually got listed on the character page, which had to be divided up specifically because of this.