KnativeServing creates 2 activators by default.

activator-pdb PodDisruptionBudget is created with "minAvailable: 80%", which means it won't allow any disruptions with just 2 activators.

No allowed disruptions will block OCP/node upgrades.

spec:
  minAvailable: 80%
  selector:
    matchLabels:
      app: activator
status:
  currentHealthy: 2
  desiredHealthy: 2
  disruptionsAllowed: 0
  expectedPods: 2
  observedGeneration: 1

https://github.com/openshift-knative/serverless-operator/blob/7f13f878e1e6ac9e11c51823349737ab8fa69c38/openshift-knative-operator/cmd/operator/kodata/knative-serving/0.18.2/2-serving-core.yaml#L2098-L2112