spec.replicas:

ApiVersion: apps/v1
kind: Deployment
metadata:
  name: spread-host-az
  labels:
    app: web-server
spec:
  replicas: 4
.../...

Related

• kubectl describe deployment | grep "Replicas:"

See also

• Kubernetes deployments, strategies: kind: Deployment, Strategies: Recreate or RollingUpdate, kubectl [ create | describe | get | scale | rollout | deployment/ ], Kubernetes Canary Deployments, deployment.kubernetes.io