Disclaimer: Like everything in this thread this post is also speculative.
So Data for Azeroth does a zone scan for people who have it installed. It also has an armory scan (I think).
Based on current estimates for retail across all regions we’re at 1.22 million scanned accounts BUT uptake of the addon in TW and KR is very low (only 57000 and 38000). Between these accounts there are about 3.5 million characters.
In those accounts there are only 38.9% of these people having actively played in the last week. So in total we’ll likely put ‘active’ retail world wide players around 450k across all regions (with likely more in CN/KR/TW - unknown amounts).
Now classic has a number of subsections and no scanner - we will mostly go off logs here listed on Ironforge.
Cata unique character raid logs in the last week: 102996
Classic Era: 11839
SoD: 71732
Anniversary Realms: 93168
Puts total classic unique character logs at 280000 over the last week. Assuming an overlap of about 10% (guesstimate) would put active players around 600-700k across all areas and regions of WoW.
Numbers higher are likely mistaking characters for accounts which is sometimes a metric for advertisement but atypically players only will play 1-3 characters on a regular basis in retail and 1-2 in classic servers.