Why is activemq not scalable?

https://github.com/GoogleCloudPlatform/click-to-deploy/blob/90829aed8e3ad4194883489190c9f314316b9c2b/k8s/activemq/chart/activemq/templates/activemq-deployment.yaml#L12

This line details how the solution is not "scalable" why? What would happen if I set it to 2? What would need to be done to get it be scalable?