The term likely refers to a specific scene or count in Chapter 28 of the manga, where Ruka's backstory is detailed through a count of heartbeats.

In the series Rent-a-Girlfriend ( Kanojo, Okarishimasu ), is a 15–17-year-old high school student who works as a rental girlfriend. Her character is defined by a chronic medical condition that makes her heart beat significantly slower than average, a condition often identified as bradycardia or congenital arrhythmia. The Medical Condition and "32x"

Ruka Sarashina [Rent-A-Girlfriend] by @Naomi_Nona - CAIBotList