In preparation of batched execution changes, remove this redundant interface. We always have other ways of getting the total shard count and even assert that they are equivalent. No need to have a redundant interface method like this around before extracting an interface from this class.
Also, remove the exception throwing that really should be assertions if anything. There's no point to this stuff, we neither expect nor properly handle exceptions in phase constructors. IF we have a bug here in the sizes of the results structures, we'll find out soon enough anyway.