int getPartitionCountForSystemStream(SystemStream systemStream) {
    return systemStreamToPartitions.getOrDefault(systemStream, DEFAULT_PARTITION_COUNT);
  }
Fetch partition count for the given input stream stream.

@param systemStream input system stream

@return the partition count if available or {@link InMemoryManager#DEFAULT_PARTITION_COUNT}
samza-core/src/main/java/org/apache/samza/system/inmemory/InMemoryManager.java