In general, the number of dgraphzero instances can be different from the number of dgraph instances. We recommend having at least 3 dgraphzero instances.
Having more dgraph instances should spread the load so that each instance is doing less work, and therefore helps to increase throughput. You can also add replication as well, so that two (or more) dgraph instances are serving the same data.
There’s some documentation here in case you didn’t see it. https://docs.dgraph.io/deploy/#multiple-instances