The original Seven Soldiers were Green Arrow and Speedy, Star-Spangled Kid and Stripesy, Vigilante, Shining Knight, and the Crimson Avenger and Wing. Wing, the Avenger's sidekick/chauffeur, was the "unofficial eighth soldier", even though there were two sidekicks as full members. When Green Arrow was removed from the line-up Post-Crisis, and replaced with the sidekickless Alias the Spider, Vigilante's sidekick Stuff, the Chinatown Kid was added, so Wing was still the unofficial eighth soldier.
The Three Stooges — there were actually 6 of them, though only 3 in any given short. (Larry, Moe, Curly, Shemp, Curly Joe, and Joe)
There are actually four musketeers in The Three Musketeers: d'Artagnan becomes one pretty early in the story, albeit assigned to a company different from the other three.
Live Action TV
On The Late Show with David Letterman's "Small Town News" segment, one of Dave's self-described favorite categories of news is things like "Classical Trio performs" with a picture of 4-6 members.
Inverted. Blake's 7 never had more than six human members. You had to include one or more of their sentient computers to bring the group up to seven.
The Grammy-Award-winning Girl Group Salt-n-Pepa consisted of Salt, Pepa, and DJ Spindarella - who I guess was the 'n'?
Similarly, Run-D.M.C. constisted of Run, DMC, and Jam Master Jay.
The band "The 4 of Us" has had five members at some points in its history.
The Thompson Twins, a British New Wave pop band, had three members at the height of their success, and as many as seven in some years.
Mythology and Religion
The Four Gods in Chinese Mythology actually has five divine beasts. The fifth one is Huang Long, the Yellow Dragon that governs the element of Earth and the direction of center. He's absent in the Japanese version, where the center is associated with void instead.
When the Big Ten Conference of the NCAA added Penn State as a member in the 1990s, they became a group of 11 schools rather than 10 (the logo they adopted after Penn State joined acknowledged this with an "11" hidden in the negative space under the arms of the T in "Ten"). The University of Nebraska became their 12th member more recently, and in 2014 it will expand with Rutgers and the University of Maryland—but they are still called the Big Ten.
And the Big 12 Conference currently has... 10 member schools.
The Atlantic 10 Conference has an ever-changing number of teams greater than ten.note Since 1995, it has had (in order) 12, 11, 12, 14, 16, and now 13 members. In July 2014, it will go back to 14.
The A10 inverted this trope from 1991 to 1995. Despite the "Atlantic 10" name, It had 9 members during this period, except in the 1992–93 school year when it had only 8.
The Pacific-12 Conference is an aversion, as it's able to change its name whenever it gets new members. It was the first the Pacific-8 when it only consisted of schools in California, Oregon, and Washington, changed to Pacific-10 upon getting Arizona and Arizona State, and changed to its current moniker with the additions of the University of Colorado and the University of Utah in 2010.
Kingdom Hearts: 358/2 Days introduces Xion, the fourteenth member of Organization XIII. She isn't actually a member. She's not even a Nobody, just a manifestation of Sora's memories. By the end of the game, she was Ret Gone'd from everyone's memories.
The Septentriones (septem = seven) in Devil Survivor 2. There are eight of them. The eight one, Alcor, is the only one not presented as a mindless abomination. He is the true identity of the Anguished One. He is the one that created Nicaea so that humanity can fight their fate of being erased from existence. In one of the routes, he even become a party member.
In EarthBound, the Runaway Five apparently consists of six members onstage, though there are only five of them when you speak with them backstage. Averted in the original Japanese, where they were the non-numerical Tonzura Brothers.
A Popeye cartoon "Jeopardy Sheriff" in which Popeye starts to read Swee'Pee the story of the Three Bears: Moe, Sam, Lefty, and George. Also referred to as "The three bears of which there were four".
The Pleiades Star Cluster are named after the seven Pleiades sisters from Classical Mythology. However, there are actually nine bright stars in the cluster (the other two are named after their parents, Atlas and Plione). What's more, the star Sterope is actually two close stars sharing the same name. Finally, a look through even a low-power telescope shows way more than 7 stars.