Version 13.0 disabled the exact size metrics (local+clustered+container NumberOfEntries+NumberOfEntriesInMemory) in the default configuration (accurate-size="false". We need replacements that are not cheap(er) to compute, so they can be enabled in the default configuration.

A first stab at the names is
approximateEntries – number of entries in the cache
approximateInMemoryEntries – number of entries in memory
approximateUniqueEntries – number of unique entries in a clustered cache

approximateEntries and approximateInMemoryEntries make sense both in the local stats and in the clustered stats (possibly counting each entry numOwners times).
approximateUniqueEntries only makes sense in the clustered stats.