Question: How many prison gangs are there on average?

Prison gangs in the United States

A prison gang is an inmate organization that operates within a prison system, that has a corporate entity, exists into perpetuity, and whose membership is restrictive, mutually exclusive, and often requires a lifetime commitment.[1] In "The Social Order of the Underworld: How Prison Gangs Govern the American Penal System," the author David Skarbekargues the emergence of prison gangs are due to the dramatic increase in the prison population and inmate's demand for safety. Skarbek observes that in a small, homogeneous environment, people can use social norms to interrupt what behavior is acceptable, but a large, heterogeneous setting undermines social norms and acceptable behavior is more difficult to determine. Prison gangs are geographically and racially divided, and about 70% of prison gang members are in California and Texas.[2] Skarbek suggests prison gangs function similar to a community responsibility system. Interactions between strangers are facilitated because you do not have to know an individual's reputation, only a gang's reputation. Some prison gangs are transplanted from the street. In some circumstances, prison gangs "outgrow" the internal world of life inside the penitentiary, and go on to engage in criminal activities on the outside.[3] Gang umbrella organizations like the Folk Nation and People Nation have originated in prisons.[4]
Prison gangs
Hispanic
La Eme or the Mexican Mafia: (Blue) "Eme" is the Spanish name of the letter "M" and it is the 13th letter in the alphabet. The Mexican mafia is composed mostly of Hispanics, although some Caucasian members exist. The Mexican Mafia and the Aryan Brotherhood are allies and work together to control prostitution, drug running, weapons and "hits" or murders. Originally formed in the 1950s in California prisons by Hispanic prisoners from the southern part of that state, Eme has traditionally been composed of US-born or raised Hispanics and has retained ties to the Southern California-based "Sureños". During the 1970s and 1980s, Eme in California established the model of leveraging their power in prison to control and profit from criminal activity on the street.
Nuestra Familia ("our family" in Spanish):(Red) The "N" is the 14th letter in the alphabet which is used as their symbol along with the Roman numeral "XIV" to represent their gang; another mostly Hispanic prison gang that is constantly at war with La Eme and was originally formed from Northern-California or rural-based Hispanic prisoners with ties to "Norteños" of Northern California opposing the domination by La Eme, which was started by and associated with Los Angeles gang members.  The gang was first established in Soledad prison in California in the 1960s.[5]
The Texas Syndicate: A mostly Texas-based prison gang that includes mostly Hispanic members and does (albeit rarely) allow non-Hispanic Caucasian members. The Texas Syndicate, more than La Eme or Nuestra Familia, has been associated or allied with Mexican immigrant prisoners, while Eme and Familia tend to be composed of and associate with US-born or raised Hispanics.
Ñetas: a Hispanic (mainly Puerto Rican) gang, found on Puerto Rico and on the eastern coast of the US.  Originally formed in 1970 in the Río Piedras State Penitentiary, Puerto Rico.[6]
Latin Kings, established in Chicago in 1954
African American
Most African-American prison gangs retain their street gang names and associations. These commonly include Rollin' sets (named after streets, i.e. Rollin' 30s, Rollin' 40s etc.) that can identify with either Blood or Crip affiliations. The Black Guerrilla Family represents an exception, as an originally politically based group that has a significant presence in prisons and prison politics.  It was founded in 1966 at San Quentin State Prison, California by former Black Panther member George L. Jackson.[7]
United Blood Nation: an African-American prison gang found on the east coast. They are rivals with the Ñetas and have ties with the Black Guerilla Family.
Folk Nation: Found in Midwestern and Southern states, allied with Crips, bitter rivals with the People Nation.
People Nation: Found in Midwestern and Southern states, allied with Bloods, bitter rivals with the Folk Nation.
D.C. Blacks: Found in Washington D.C. by African-American inmates, are allied with the Black Guerilla Family and United Blood Nation, and enemies to the Aryan Brotherhood and Mexican Mafia.
Conservative Vice Lords (CVL): A primarily African American gang that originated in the St. Charles Illinois Youth Center outside Chicago.[8] In Chicago, CVL operated primarily in the Lawndale section and used drug sales profits to continue operation and used prisons to train and recruit new members.[9]
Caucasian
Aryan Brotherhood: A white prison gang that originated in California's San Quentin Prison amongst White American prisoners in 1964. Their emblem, "the brand", consists of a shamrock and the number 666. Other identifiers include the initials "AB", Swastikas and the sigrune.[10]  Perhaps out of their ideology and the necessity of establishing a presence among the more numerous African American and Hispanic gang members, the AB has a particular reputation for ruthlessness and violence. Since the 1990s, in part because of this reputation, the AB has been targeted heavily by state and federal authorities. Many key AB members have been moved to "supermax" control-unit prisons at both the federal and state level or are under federal indictment.
Nazi Lowriders: A newer white prison gang that emerged after many Aryan Brotherhood members were sent to the Security Housing Unit at Pelican Bay or transferred to federal prisons. NLR is associated with members originally from the Antelope Valley and is known to accept some light-skinned or Caucasian-identified Hispanic members.
Public Enemy No. 1: A white street and prison gang based in Southern California. They have replaced the NLR in holding the "keys" for the Aryan Brotherhood on the mainline prison population.
Dirty White Boys: A white prison gang made up of inmates from Texas, and have a heavy presence in the federal system.
European Kindred: a white supremacist prison gang founded in Oregon that is affiliated with the Aryan Brotherhood and the Ku Klux Klan.
Confederate Knights of America: a white supremacist prison gang in Texas that is affiliated with the KKK and AB.
Aryan Circle: a white supremacist prison gang concerned about race before money.
Dead Man Incorporated (DMI): a predominantly white prison gang founded in the Maryland Correctional System with branches in many other correctional facilities throughout the U.S.
Aryan Brotherhood of Texas: Despite the similarity of the name, the Aryan Brotherhood of Texas (A.B.T.) does not have ties with the original Aryan Brotherhood. Founded in Texas in the 1980s the A.B.T. was created mainly as a criminal enterprise.[11]
Brotherhood of Aryan Alliance (a. k. a. the "211's") [12]
Simon City Royals: A street gang established in Chicago during the late 1950s.
Latent prison management function
Christian Parenti argues in his book Lockdown America that prison gangs serve a convenient function for the prison establishment and officers. They help regulate rogue and rebellious elements within the prison population without intervention from prison authorities.[13]
Parenti sees the repression dished out by gangs on non-affiliated prisoners as a latent function of prison gangs. Thus, gangs are often more-or-less tolerated by prison administrators due to the side-benefits they afford.
US prison gangs in fiction
Oz
Blood in Blood out
American Me
American History X
Felon
Shot Caller
See also

Incarceration in the United States
Gangs in the United States