While Topps has never publicly disclosed a total production count for uncut sheets featuring only card backs, these items are exceptionally rare and typically categorized as progressive proofs or test sheets from the printing process.
The following known "Backs Only" uncut sheets have surfaced in the hobby:

Up for sale here is a very rare:
YEAR: 2019
BRAND: Topps Chrome
SPORT: Baseball
DESCRIPTION: PROGRESSION PROOF UNCUT SHEET (BACK ONLY)
FULL OF STARS!!!

1. Mike Trout
2. Ronald Acuna Jr.
3. Albert Pujols
4. Juan Soto
5. Clayton Kershaw
6. Alex Bregman
7. Starling Marte
8. Walker Buehler
9. Francisco Lindor
10. Zach Greinke
11. Nick Castellanos
13. Josh Bell
14. Jacob Degrom
15. Trevor Story
16. Max Sherzer
17. Kris Bryant
18. Luis Severino
19. Cody Bellinger
20. Max Kepler
21. Aaron Nola
And many others!
S+H is $18!!!
Thanks for looking!!!