<p>In this tutorial <a href="https://kubernetes.io/docs/tasks/run-application/run-replicated-stateful-application/" rel="nofollow noreferrer">https://kubernetes.io/docs/tasks/run-application/run-replicated-stateful-application/</a>, the replicas mentioned in the stateful is 3 and 3 pods get started.</p>
<pre><code>apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mysql
spec:
  selector:
    matchLabels:
      app: mysql
  serviceName: mysql
  replicas: 3
...
</code></pre>
<p>What does 2/2 mean in the output of the command <code>kubectl get pods -l app=mysql --watch</code>:</p>
<pre><code>NAME      READY     STATUS    RESTARTS   AGE
mysql-0   2/2       Running   0          2m
mysql-1   2/2       Running   0          1m
mysql-2   2/2       Running   0          1m
</code></pre>

<p>You have 2 containers in a pod, it means both of them are running.</p>
