No real answer for you, but here's what I see on browser*:
I have never 'cleaned up' my inbox in either city. I just let all msgs get pushed off the bottom. I am Eastern time in the US.
Ch17 city (city created in March 2017)
I have 18 pages with 5 msgs per page and there are 2 msgs on page 19. The date/time of the last post in the last msg on p 19 is: Oct 16, 2021, 9:20pm. Total msgs: 92
Ch16 city (city created in May 2018)
I have 6 pages with 5 msgs per page and there are 4 msgs on page 7. The date/time of the last post in the last msg on p 7 is: Oct 19, 2021, 5:36pm. Total msgs: 34
*I was flipping between tabs and typing as I went; I had never checked it before and there's a stark difference in the 2 cities! It's understandable I have more msgs in the first city: I'm AM there as well as FA coordinator. I send lots of FS wide msgs during the FA and get more direct msgs since I became AM last spring than when I was just a Mage. We also have 6 AWKP swap threads in that FS that get renewed about every 100 msgs or so when they get slow to load for some members. The other city, we have one NetZero thread for all AWKP swaps. There are few FA msgs there as we all use the same spreadsheet with color coded sections for badges for the different paths, so it's easier to tell where we are/where we're going.