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The term concept limit here refers to a maximum number of discrete, encyclopedically-valid concepts in the real world - such that require an article for describing them, and beyond which there is nothing to write about.
English Wikipedia's success has meant that it has grown so quickly as to make some people wonder if it could be approaching this limit - such that its historical modality of rapid changes raises issues such as the slowing of growth and the deletion of existing poor-quality work.
Conversely, WP:FOUR is a good point raised by those questioning this viewpoint, as none of these should be theoretically possible, and yet they just keep coming.