As of last night's Battleground 2014, John Cena and Randy Orton have faced each other 21 times on PPV.
9 of those matches have been singles matches, with Orton leading 5-4:
1) SummerSlam 2007 (Cena)
2) Unforgiven 2007 (Orton)
3) No Way Out 2008 (Cena)
4) SummerSlam 2009 (Orton)
5) Breaking Point 2009 - I Quit Match (Cena)
6) Hell in a Cell 2009 - Hell in a Cell Match (Orton)
7) Bragging Rights 2009 - Anything Goes 60-Minute Iron Man Match (Cena)
8) TLC 2013 - TLC Match (Orton)
9) Royal Rumble 2014 (Orton)
Cena and Orton have also faced each other in TWELVE additional PPV matches involving others. Cena is 6-6 in those matches (Orton is 4-8)
1) Backlash 2007 - Fatal Four Way Match: John Cena beat Shawn Michaels, Edge, and Randy Orton
2) Vengeance 2007 - WWE Championship Challenge: John Cena beat Mick Foley, King Booker, Randy Orton, and Lashley
3) WrestleMania XXIV - Randy Orton beat Triple H & John Cena
4) Backlash 2008 - Triple H beat Randy Orton, John Cena and JBL
5) Night of Champions 2009 - Randy Orton beat Triple H & John Cena
6) Elimination Chamber 2010 - Elimination Chamber: John Cena beat Sheamus, Triple H, Kofi Kingston, Ted DiBiase, and Randy Orton
7) Fatal Four Way 2010: Fatal Four Way: Sheamus beat Champion John Cena, Randy Orton, and Edge to win the title
8) Night of Champions 2010: Six Pack Elimination Match: Randy Orton beat Champion Sheamus, John Cena, Wade Barrett, Edge, and Chris Jericho
9) Elimination Chamber 2011: John Cena beat CM Punk, John Morrison, Randy Orton, Sheamus, & R-Truth
10) Elimination Chamber 2014: Randy Orton beat Daniel Bryan, John Cena, Cesaro, Christian and Sheamus
11) Money in the Bank 2014: John Cena beat Kane, Randy Orton, Roman Reigns, Alberto Del Rio, Sheamus, Bray Wyatt and Cesaro
12) Battleground 2014: John Cena beat Randy Orton, Kane, & Roman Reigns
There have been 96 PPVs from Backlash 2007 (their first PPV match) and last night's Battleground 2014. This means roughly 1 out of every 5 WWE PPVs during that time has featured some sort of Cena/Orton match.