https://www.thenerdmag.com/rumor-bli...e-for-azeroth/
First off, we must emphasize that this is not an official news. This was rumored when a new API for develope.battle.net was launched. Allegedly the new API was leaking player subscription end dates among other info. It was then reported that at the start of Battle for Azeroth, the sub count was somewhere near 3.2 million. Right now the sub count is 1.7 million which is 1.5 million less than the initial count. It is further alleged that 25% of the current 1.7 million are secondary purchases which is 425,000.
Source is Weak Aura's twitter account.