Apart from a few Immortal Sects which keep their Mountain Gates wide open, allowing anyone who is not too lacking to join,
most Immortal Sects only accept disciples with superior qualifications; even those may not be enough, as stringent requirements of character must be met before one can be admitted.
For many Immortal Sects, superior qualifications are the ticket to entry.
Whether more conditions are added depends on how plentiful the sect's resources are and how strong it is.
The more resources and strength an Immortal Sect has, the stricter the requirements for recruiting disciples, resulting in fewer members.
Take the Qing Immortal Sect, for example, which suppresses the entire Changqing Domain. Even though it's now in decline, it still occupies twenty-one countries within the domain and remains the undisputed hegemon.
Yet, such a Great Immortal Sect only has, in total, three hundred true disciples and five thousand sect members.