code: public synchronized void addConsumer(Consumer consumer, ConsumerConfiguration configuration, int instances) {
          for (int i=0; i < instances; i++) {
              this.consumerHolders.add(new ConsumerHolder(consumer, configuration));
          }
      }
docs: Adds a consumer to the container and configures it according to the consumer
  configuration. Does NOT enable the consumer to consume from the message broker until the container is started.
  
  <p>Registers the same consumer N times at the queue according to the number of specified instances.
  Use this for scaling your consumers locally. Be aware that the consumer implementation must
  be stateless or thread safe.</p>
  
  @param consumer The consumer
  @param configuration The consumer configuration
  @param instances the amount of consumer instances