Horizontal scaling based on the number of events published in NATS server
You could use Keda autoscaler. Keda is a CNCF project that can scale your deployments based in different streaming/queue services, there is a implemented autoscaler for NATS: https://keda.sh/docs/1.5/scalers/nats-streaming/
I've been using Keda autoscaler with rabbitmq and works like a charm.