code: public int countCurrentDockerSlaves(final DockerSlaveTemplate template) throws Exception {
          int count = 0;
          List<Container> containers = getClient().listContainersCmd().exec();
  
          for (Container container : containers) {
              final Map<String, String> labels = container.getLabels();
  
              if (labels.containsKey(DOCKER_CLOUD_LABEL) && labels.get(DOCKER_CLOUD_LABEL).equals(getDisplayName())) {
                  if (template == null) {
                      // count only total cloud capacity
                      count++;
                  } else if (labels.containsKey(DOCKER_TEMPLATE_LABEL) &&
                          labels.get(DOCKER_TEMPLATE_LABEL).equals(template.getId())) {
                      count++;
                  }
              }
          }
  
          return count;
      }
docs: Counts the number of instances in Docker currently running that are using the specified template.
  
  @param template If null, then all instances are counted.