Passing replicas number into container

I suggest you try to pass this information as an update to the statefulset.

See this for reference: https://kubernetes.io/docs/tutorials/stateful-application/basic-stateful-set/#updating-statefulsets

There are two ways either increase the number in your DeploymentConfig directly or use oc scale statefulset <name_of_statfulset> --replicas=2

apiVersion: apps.openshift.io/v1
kind: DeploymentConfig
metadata:
  labels:
  app: jekyll
  name: jekyll
  namespace: test
spec:
  replicas: 1 <----------- here increase the number
 

If you launch the statefulset with the following key set

{"spec":{"template":{"spec":{"serviceAccountName":"myserviceaccount"}}}}

you can run

curl 
https://kubernetes/apis/apps/v1/namespaces/<NAMESPACE>/statefulsets/<STATEFULSET> 
-k -H "Authorization: Bearer $(cat /var/run/secrets/kubernetes.io/serviceaccount/token)" 
| jq '.spec.replicas'

to extract the replica count. This, of course, relies on the service account (myserviceaccount in this example) to have appropriate permissions to actually query the k8s API for this information, as well as having curl and jq available in the container.

You could then setup some kind of background job in the container to regularly make this request and update an env var or file or whatever.

Define the number of replicas in the values.yaml and pass it anywhere in resources (statefulset)

Update 1: You can neither have a dynamic environment variable nor refer to the replicas field in STS. So the only way to get an updated count of STS in each pod is to redeploy with the new value of replicas in values.yaml:

replicas: 3

Update 2:

According to this link, you can mount a secret now to each pod and stay up to date about replicas count.