Question: Every so often while shortwave listening I’ll come across a frequency where tons of hams are talking all over each other giving a call sign and a six digit number only (not a QSO). I imagine these numbers may be club membership numbers but I can’t figure out the purpose. Is this some way to build up a large number of QSOs? Doesn’t seem like the requirements of a QSO are met by just getting a call sign and number. And how on earth does anyone copy all of these contacts given that everyone is pretty much talking over themselves, voices overlapping, with no moderator or any attempt to direct traffic?

What makes this even weirder is that I’ll sometimes hear other hams on the band who mention this but don’t know what it’s for either.

Answer: 
Guessing it was a contest, and the numbers were the exchanges, per the contest rules.

The exchanges vary by contest, so it depends on which contest you heard.

Try searching the contest-calendar, here:

<https://www.contestcalendar.com/weeklycont.php>

Perhaps with some more information we can narrow it down.
If it was on the 10m band then it could have been the 10-10 club net.https://www.ten-ten.org/