that the game has accumulated a total of five million paid subscribers during its 21 months on the market.
The five million subscriber total doesn't include trial accounts, and only those who have at one time or another paid the $12.99/$14.99 monthly fee making it a substantial feat.
The number of unique subs over the time period means nothing. What does count is the number subbing at any one given point in time. Most people try MMOs and then never play them again. An MMO needs regular players.

These days a sub based MMO is an anomaly. What are the others as I can't think of any that are left. Mind you SE are pushing those extra payments and it doesn't make me a happy kitty.