Manual:$wgMemCachedServers

Array of memcached network servers, or unix socket pathnames using the PHP form (example) (for use with the memcached memory-based object store).

Note that beginning in MediaWiki 1.35.6, 1.36.4 and 1.37.2, when specifying a unix socket pathname, a trailing  must not be specified. It was ignored by the prior versions, but is now treated as part of the pathname.

Details
To use multiple servers (physically separate boxes or multiple caches on one machine on a large-memory x86 box), just add more items to the array. To increase the weight of a server (say, because it has twice the memory of the others and you want to spread usage evenly), make its entry a subarray:

You can have multiple wikis point to the same Memcached server, as long as each have different wiki-ids. Certain cache keys are intentionally shared in such a scenario, such as rate limiting stuff.

More information in.