And the admin has also spoken against it already so we should not waste time discussing it.
Coming back to your suggestion OP, it is not practical as :
1) It hampers the growth of the forum. Forums spend huge money on promotion to get new members and you want this forum to limit the number of members ?
2) Someone mentioned above that if there are limits like 2000 members per month, it would be bots competing for those accounts and not real people.
3) What do you think will happen to those who fail to join here ? They will got to alternative forums thus more competition for this forum.
There can be other ways to control bot accounts like requiring users to make "n" number of posts per week or month. This will have to be implemented in wisely to prevent those posts from being automated as well.