As has often been explained elsewhere, a new server is opened only when the current servers are no longer fulfilling the requirements of current players.
The quickest way for a current player to achieve this is for them to actively request friends who might be unaware of the game to consider playing.
when many players are on a server at once, it causes a measurable slowing of how fast each player action is processed.
once this 'lag' reaches a certain amount for the servers current available, a new server may be considered as needed by the company...
since I am not one of the developers, I don't have access to the figures, but this is how server/software providers are known to operate...
I cannot estimate when the company will decide a new server will be needed, but rest assured that when it is, one will most likely be added.
Thank you for showing your interest in improving the game.