These are the ones listed in Wikipedia as compilations. I did other searches but these are all I found. Hard to imagine there's almost double this amount.
Showdown
Olé ELO
The Light Shines On
Three Light Years
The Light Shines On Vol 2
ELO's Greatest Hits
A Box of Their Best
Four Light Years
Afterglow
ELO's Greatest Hits Vol. 2
The Very Best of the Electric Light Orchestra
Strange Magic
Light Years
Definitive Collection
Flashback
The Ultimate Collection
The Essential Electric Light Orchestra
All Over the World
Ticket to the Moon
Actually you missed quite a bit on Wikipedia!
Electric Light Orchestra discography - Wikipedia, the free encyclopedia
Compilation albums
Chart and Certificated compilations
1976 Olé ELO
1977 The Light Shines On
1978 Three Light Years
1979 ELO's Greatest Hits
1989 The Very Best of The Electric Light Orchestra
1994 The Very Best of the Electric Light Orchestra
1997 Light Years, The Very Best Of Electric Light Orchestra
2001 The Ultimate Collection
2005 All Over the World: The Very Best of Electric Light Orchestra
Complete list
Showdown (1974)
Olé ELO (1976)
The Light Shines On (1977)
Three Light Years (1978)
The Light Shines On Vol 2 (1979)
ELO's Greatest Hits (1979)
A Box of Their Best (1980)
Four Light Years (1980)
The Best of ELO (1981)
Eldorado/Electric Light Orchestra II (cassette only reissue) (1983)
A Perfect World of Music (1985)
First Movement (1986)
The Very Best of The Electric Light Orchestra (1989)
Afterglow (1990)
ELO Classics (1990)
Early ELO: 1971-1973 (1991)
ELO's Greatest Hits Vol. 2 (1992)
Burning Bright (1992)
The Very Best of the Electric Light Orchestra (1994)
Strange Magic: The Best of Electric Light Orchestra (1995)
The Best of Electric Light Orchestra (1996)
Light Years, The Very Best Of Electric Light Orchestra (1997)
The Danish Collection, The Very Best of Electric Light Orchestra (1997)
The Swedish Collection, The Very Best of Electric Light Orchestra (1997)
Friends & Relatives (1999)
Definitive Collection (1999)
World Ballads: Electric Light Orchestra (1999)
Flashback (2000)
The Ultimate Collection (2001)
The Essential Electric Light Orchestra (2003)
Early Years (2004)
All Over the World: The Very Best of Electric Light Orchestra (2005)
Harvest Showdown (2005)
The Collection (2006)
The Harvest Years: 1970-1973 (2006)
Ticket to the Moon: The Very Best of Electric Light Orchestra Volume 2 (2007)
ELO: Los Monstruos del Rock (2008)
Playlist: The Very Best of Electric Light Orchestra (2008)
The Very Best of Electric Light Orchestra (2011)