There are 20 Jay-and-Acacia missions co-written by Jay and Acacia:
TOS chapter 1 – 9, 11 – 18 and 20 – 22.

TOS chapter 10 "Vacation at OFUM" is not really a mission; it’s more like an interlude.

TOS chapter 19 "Torment" is a Jay-and-Death mission, co-written with Miss Cam

TOS chapter 23 "To Know Where You Are Going" was apparently abandoned by Jay and Acacia and then finished by Jay and Rhysdux.

TOS chapter 24 "A Taste of Blood" is a Jay-and-Dee mission probably co-written by Jay and whoever wrote Dee.

TOS chapter 25 "Broken Doll" and chapter 26 "No Way" are Jay-and-Rhus missions co-written by Jay and Rhus Radicans’s author Thalia Weaver.

HG

That those are the only ones that have hit 20 plus missions.

Like, I'm pretty sure it just means that there's only a select group of people who've even come close to TOS's count. =]