I expect around a million subscribers for classic launch, maybe more but many will quit after the first month.
It'll peak at BWL patch with the serious players (ignoring the tourists quitting after the first few weeks) and then start to really dive as people will get burnt out due to MC/BWL farming and PvP rank grind. (Most will be burnt around R10-12)
Population will most likely half until AQ gates and then resurge before taking another dive after a month into AQ.
It'll be in a steady decline until naxx release. I don't see many coming for naxx-pre patch as its not as significant as the AQ gates.
Re-surge at naxx before quickly diving again as most people aren't getting into naxx.
Towards the end of the classic cycle until Classic+ or new expansion is in discussion I can see a pop around 100k maybe a little lower across all realms/timezones. Which is cool still. I'm just going off how every launch for private servers are like. Humans tend to do the same thing no matter the scale so I feel this is pretty accurate.