postBuild:
    substitute:
      quote: '"' # double quote var
      replicas: "1"

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
....
patches:
  - patch: |-
      apiVersion: apps/v1
      kind: StatefulSet
      metadata:
        name: worker
      spec:
        replicas: ${quote}${replicas}${quote}$
        ....