In the hip hop world this is known as a "posse cut," which is a song that features at least four consecutive verses by a different rapper. For example, "The Anthem" by Sway & King Tech, featuring RZA, Tech N9ne, Eminem, Xzibit, Pharoahe Monch, Kool G Rap, Jayo Felony, Chino XL, and KRS-One.
Pigface is the side project of over one hundred musicians. While their individual tracks typically don't have a massive number of musicians, the credits to their albums are quite staggering, as most of them feature well over a dozen performers.
Blake Shelton's 2013 single "Boys 'Round Here" features a huge cast of characters: co-writers Dallas Davidson and Thomas Rhett; producer Scott Hendricks; the Pistol Annies (a side project founded by Shelton's wife, singer Miranda Lambert, along with Ashley Monroe and Angaleena Pressley); and RaeLynn, a contestant on The Voice, on which Shelton is a judge.
His 1988 single "Young Country" featured several up-and-comers and also-rans: Butch Baker, Steve Earle, Highway 101, Dana McVicker, Marty Stuart, Keith Whitley, and T. Graham Brown.
Similarly, Travis Tritt's 1992 single "Lord Have Mercy on the Working Man" had backing vocals from Brooks & Dunn, T. Graham Brown, George Jones, Little Texas, Dana McVicker, Tanya Tucker, and Porter Wagoner.
The multi-artist album A Tribute to Tradition (1998) featured the new song "Same Old Train", featuring Clint Black, Joe Diffie, Merle Haggard, Emmylou Harris, Alison Krauss, Patty Loveless, Earl Scruggs, Ricky Skaggs, Marty Stuart, Pam Tillis, Randy Travis, Travis Tritt, and Dwight Yoakam. This recording won the 1999 Grammy award for Country Vocal Collaboration.
In July 2001, an all-star version of "America the Beautiful" made the country charts (and re-entering as a patriotic tribute after 9/11) featuring Trace Adkins, Billy Dean, Vince Gill, Carolyn Dawn Johnson, Toby Keith, Brenda Lee, Lonestar, Martina McBride, Jamie O'Neal, Kenny Rogers, and Keith Urban.
In 1967, all of the artists on RCA's country music division paid tribute to Chet Atkins with the song "Chet's Tune". The recording featured Eddy Arnold, Bobby Bare, Don Bowman, Jim Ed Brown, Archie Campbell, Floyd Cramer, Skeeter Davis, Jimmy Dean, George Hamilton IV, Homer & Jethro, Waylon Jennings, Hank Locklin, John D. Loudermilk, Willie Nelson, Norma Jean, Jerry Reed, Connie Smith, Hank Snow, Porter Wagoner, and Dottie West.
Many Charity Motivation Songs are massive gatherings of famous singers, "We Are the World" being the most famous example.
The credits to Mike Watt's first solo album practically reads like some kind of Alternative Rock hall of fame thanks to collaborations with literally dozens of musicians with their own bands. To list everyone who collaborated on the album would create a massive wall of text, so just take a look at his touring band at the time: Watt, himself, on bass and vocals, Eddie Vedder on vocals and guitar, William Goldsmith on drums, DaveGrohl alternating between drums and guitar, and PatSmear on lead guitar.
Urban/contemporary Christian artist Kirk Franklin's 1998 song "Lean On Me" features guest vocals by Bono, Mary J. Blige, R. Kelly, and Crystal Lewis. A remix of the song was included as a Hidden Track on the album The Nu Nation Project; the remix features additional vocals by Boyz II Men, Xscape, En Vogue, New Edition, Destinys Child, K-Ci and Jojo, and Divine.