For each 10 million items in the search index, you have to add a new index partition. When you add a new index partition, the search index has to be re-partitioned. Depending on the size of the search index, this re-partitioning can take several hours to complete.
You add an index replica to the search topology to achieve fault tolerance for an existing index partition. You place the index replicas on separate failure domains on separate servers. When you add an index replica, you add a new index component to the search topology and associate it with the index partition that you want to make a replica of.