Non-negative integer less than or equal to 50; optional

Specifies the number of parallel shards to use to run the test.

This value will override any heuristics used to determine the number of parallel shards with which to run the test. Note that for some test rules, this parameter may be required to enable sharding in the first place. Also see --test_sharding_strategy.

Sharding requires the test runner to support the test sharding protocol. If it does not, then it will most likely run every test in every shard, which is not what you want.