Several leagues have official attendance numbers, although some are not recent.
Here are official numbers:
We made estimates of average numbers based on the capacity of the arenas in the league. Then we took our criteria "Status of Basketball" in the respective country and substracted a certain number of % from the arena capacity. For example the German BBL has an above average status of Basketball in the country (4) and 85% of the arenas capacities were filled in the season.
For status 5 we estimated a 90% capacity, for status 4 80%, status 3 70%, status 2 60% and status 1 50%. For 2nd tiers we took off 10% more on each status and took another 20% off that result (3rd tiers 30%) in order to get a more realistic result.
Estimates:
Sources: Each league's English Wikipedia Page for Attendances
https://www.easycredit-bbl.de/de/statistiken/zuschauer/zuschauer-statistiken/
https://www.2basketballbundesliga.de/zuschauerzahlen-proa/
https://www.aba-liga.com/news/41950/average-attendance-increases-over-2-600-for-the-second-time/