The sentence reads:
replicas tells Gluster how many replicas to ensure exist; this number can vary
depending on how many servers you have in the brick’s cluster, and how
tolerance you have for server outages.
It should probably read "...,and how much tolerance you have..."
The phrase "...tells Gluster how many replicas to ensure exist;..." reads awkwardly. I'm not certain how that should parse. s/to ensure/should/ ?