Sweetspot for servers is between 3k and 3.5k. This number provides the perfect level stratification across the server. Anything less than 3k makes it too easy to level which results in the player base being too skewed towards level 60, resulting in server death from nothing to do and players not wanting to be ganked. Anything more than 3.5k overcrowds the server, leading to too many players and server death as migrations to emptier servers happens.